All four men aboard a Marine Corps jet that severed a gondola cable in Italy last month, plunging 20 people more than 300 feet to their deaths, have been charged with negligent homicide and involuntary manslaughter, a Defense Department official said tonight.

The official, speaking on condition of anonymity, said the crew members of the EA-6B Prowler have also been charged with dereliction of duty. If convicted, they face punishment ranging from reduction in rank -- effectively ending their careers -- to dishonorable discharges followed by several years in prison, the official said.

The Marines' decision to prefer charges, based on the recommendation of an investigation board, comes as no surprise. At a news conference in Italy on March 12, Maj. Gen. Michael P. DeLong, deputy commander of the Marine Corps Atlantic Command, said preliminary findings blamed ''crew error'' for the Feb. 3 accident and said the plane had been flown ''aggressively.''

General DeLong said further that the craft was flying well under 1,000 feet rather than the minimum of 2,000 feet, and near its top speed of 550 miles per hour, or some 60 m.p.h. over the limit in the area. The accident occurred in clear weather as the plane passed over the ski resort of Cavalese.

The Italian ski-lift operator was killed, along with tourists from Italy, Germany, Poland, the Netherlands and Austria. The accident was a serious blemish on relations between the United States and Italy, where residents had long complained about American military craft flying dangerously low over densely populated areas.

But the Pentagon official who spoke tonight said the charges lodged against the four men had nothing to do with any desire to repair relations with Italy. ''The process is the same'' whether an accident occurs in Italy or, say, Colorado, the official said.

General DeLong said his investigating team had been unable to determine why the Prowler, an electronic surveillance jet, was flying so low so fast.

One unanswered question is why the four men face identical charges, since at the March 12 news conference officials said they bore varying degrees of culpability. The official who spoke tonight said that he could not address that aspect of the case, and that it was too early to speculate on the disposition in any event. He said more details might emerge when the Marines announced the charges on Friday at its Atlantic headquarters in Norfolk, Va.

''All members of the air crew shared responsibility for the flight,'' the official said. Officials have identified the four, all captains, as Richard Ashby, 31, of Mission Viejo, Calif., the pilot; Joseph Schweitzer, 30, of Westbury, N.Y., the co-pilot; William Raney 2d, 26, of Englewood, Colo., and Chandler Seagraves, 28, of Nineveh, Ind.

The next step in the case is an Article 32 hearing, not unlike a civilian grand jury proceeding, which will determine whether there is evidence to support the charges, the official said. That will be held at Camp Lejeune, N.C., and presided over by Lieut. Col. Ronald L. Rogers, a Marine Corp judge. No date has been announced.

The conclusion from the Article 32 hearing will be forwarded to Lieut. Gen. Peter Pace, the commander of the Marines in the Atlantic, who will decide whether the four should be court-martialed, the official said.

The four defendants were on temporary duty at Aviano Air Base when the accident occurred. They have since been returned to their home base at Cherry Point, N.C., and placed on non-flying status.

A Marine Corps pilot whose plane severed gondola cables in Italy last year, sending 20 people to their deaths, was acquitted of charges of involuntary homicide and manslaughter today by a court-martial jury that found no negligence, even though the plane was flying lower and faster than rules permitted.

The pilot, Capt. Richard J. Ashby, 31, of Mission Viejo, Calif., stood at attention to hear the verdict. There was no change in his bearing as the president of the jury, Col. William T. Snow, said, ''This court finds you, of all the charges and specifications, not guilty.''

The pilot's sister, Cary Lee Horsager, seated along with about 30 spectators in the spartan military courtroom here, let out a whoop.

One of the prosecutors, Maj. Vernon S. Couch, turned to relatives of German victims seated behind him and said, ''I'm sorry.''

The verdict infuriated many Italians, from people in the streets of Rome to Prime Minister Massimo D'Alema, who was in Boston today and said he would talk about the verdict when he met with President Clinton at the White House on Friday. [Page A16.]

Captain Ashby's reaction was somber. ''Basically all I really want to say is it's been a tragedy for all involved,'' he said, ''and my heart and my thoughts and prayers go out to the families of the victims of this tragedy.''

Captain Ashby still faces a charge of obstruction of justice, as does his navigator, Capt. Joseph P. Schweitzer, 30, of Westbury, N.Y., because they have been accused of concealing a videotape on which they had recorded part of the flight and that investigators thought might have been useful in determining what had happened. The whereabouts of that tape are not known.

But after the verdict today it was not clear whether the Marines would still press homicide or manslaughter charges against Captain Schweitzer. During the flight on Feb. 3, 1998, Captain Ashby sat at the jet's single set of controls and Captain Schweitzer sat to his right and was the mission planner.

Captain Schweitzer's civilian lawyer, David Beck, said this afternoon that he would ask on Friday that all the charges be dropped.

The obstruction charge that the two men still face carries a possible one-year sentence.

Had Captain Ashby been convicted, he would have faced up to 10 years in prison for each death and would have been led directly to the brig. Instead, the judge, Col. Robert Nunley, said, ''Captain Ashby is certainly free to go.''

Captain Ashby's civilian lawyer, Frank Spinner, said today that it was too soon to say what his client would do now that he had been cleared of the most serious charges. Since the accident, the pilot had been assigned to assist in his own defense.

The 17-day court-martial, along with pretrial hearings and two Marine Corps investigations, made clear that lack of communication had been a factor in the accident.

The verdict delivered after seven hours of deliberations this morning and on Wednesday afternoon raised the possibility that the most serious punishments meted out will have been against the squadron commander, who was relieved of command, and the squadron's director of safety standardization, who received a letter of reprimand that effectively ends his career.

Relatives of the some of the victims, German tourists, were tearful and outraged today. Sindy Renkewitz, whose father and sister were on the cable car, along with her sister's fiance, said she had worried that the Marine Corps jury would not convict one of their own.

''There's a saying in Germany,'' said Ms. Renkewitz. ''It says, one crow doesn't pick out the eye of the other one.''

Rita Wunderlich said through a translator: ''I buried my husband a year ago. Today it was his second funeral.''

Ms. Wunderlich, who was not present when Captain Ashby spoke to reporters after the verdict, said: ''If Mr. Ashby really feels sorry, and he told you so, why didn't he tell us? He could have looked into our eyes but I don't know if he can do that.''

Ms. Wunderlich added that if her daughter had done something to cause 20 deaths, she did not know if she could enter the courtroom. And referring to the whoops by the pilot's sister after the verdict, she said of her daughter, ''I don't know if she were declared innocent, if I could have screamed out of joy.''

Ms. Wunderlich added: ''Every normal person would have said he's guilty. It's only the members of the jury and his defense attorneys who think that he was innocent, and I cannot understand that.''

The plane was supposed to stay 1,000 feet above the ground but struck the cables at about 360 feet. The jet was not supposed to be flying faster than 518 miles an hour, but crew members said they were unaware of that order. They acknowledged having flown at up to 644 miles an hour.

The acquittal was foreshadowed by the warning of a hearing officer who conducted an eight-day inquiry last year. In a proceeding loosely equivalent to a civilian grand jury, he said that convicting the crew would be difficult because many parties had been at fault.

A major question before the jury was whether Captain Ashby's conduct matched that of a ''reasonable pilot'' in the same or similar circumstances. Inquiries after the accident showed that hardly anybody in the squadron could describe the rules correctly and that the American charts, used by the Marines and the Air Force, did not show the ski lift.

In addition, the plane may have had problems with its radar altimeter, which measures distance above the ground, according to testimony at the court-martial. The crew, flying the plane through typically aggressive maneuvers meant to stay below the mountaintops and thus simulate radar evasion, may have been flying the plane in a way that rendered the radar altimeter useless, although they were relying on it to keep them safe, expert witnesses said.

Captain Ashby's lawyers also argued that as the valley where the accident occurred grew wider, it could have produced an optical illusion for the crew, making them believe that they were gaining altitude.

During the trial, defense lawyers presented evidence of serious problems in Marine Corps training, equipment, management and procedure, as well as certain aspects of Air Force operations. Communications between the Air Force squadron permanently stationed at Aviano Air Force Base in Italy and the Marine squadrons that rotated through were shown to be poor.

Commenting on the obstruction charge, Mr. Spinner, Captain Ashby's lawyer, said he would appeal to Lieut. Gen. Peter Pace, the Commander of Marine Forces, Atlantic, who had ordered the court-martial, to consider a less formal disciplinary procedure that would result in a smaller punishment. Hiding the tape was ''regrettable,'' he said, ''but I don't think it rises to the level of obstruction.''

A spokesman for the Marine Corps, Maj. Scott B. Jack, said this afternoon that ''the trial counsel is continuously re-evaluating the aspects of the case.''

Captain Ashby testified during the trial that it was his camera but Captain Schweitzer's tape. He said that Captain Schweitzer did not use the camera during the low-level portion of the flight, when the accident occurred, and that he had taken the tape from the plane and later given it to Captain Schweitzer.

Mr. Spinner, the pilot's lawyer, told reporters that the verdict showed that the incident was a training accident, not a crime. ''I think it's time, now that the truth has come out in the courtroom, for the Marine Corps to look back, at how this trial came about,'' he said. ''And I think it's time for Congress perhaps to look at the Marine Corps once all these issues are resolved and perhaps look at what went wrong.''

The obstruction charge was added after the original charges were filed, and Captain Ashby elected to face that charge in a second court-martial. Captain Schweitzer agreed to face all the charges at once.

Captain Ashby was also acquitted on lesser charges of destruction of Government and private property.