Minutes after crashing his car on a freeway exit ramp and watching as his best friend was thrown to his death early today, Houston Oilers defensive tackle Jeff Alm took a shotgun from the car and killed himself, apparently in a fit of guilt or despair, the police said.

While some details of the incident just after 2:30 A.M. local time were still unknown, authorities said they believed that the car crashed accidentally, after Alm was speeding and lost control of his 1993 Cadillac El Dorado. He then ran across the ramp and looked down an embankment, discovering that his boyhood friend, Sean P. Lynch, had been thrown to his death.

"It's believed he was despondent and obtained a pistol-grip shotgun," said Alvin Wright, a spokesman for the Houston police. "It's believed he fired off two rounds in the air before going to the guardrail, sitting down and shooting himself in the head."

Wright said that there were no apparent injuries to Alm apart from the gunshot wound. The police said they did not know whether either man was wearing a seat belt but believe that Lynch, at least, was not.

Alm apparently was going too fast to negotiate a curve in the ramp off Interstate 610 in southwest Houston, the police said. After the car hit the guardrail, they said, Lynch was thrown from the car and went over the rail, landing more than 20 feet below on a service road beneath the overpass. The accident was discovered by a passing motorist, who notified the police.

The Oilers said Alm was a gun collector and was legally carrying the shotgun in his car.

Alm and Lynch, both 25, grew up in the Chicago suburb of Orland Park and were teammates on the football team at Carl Sandburg High School. Though Lynch did not go on to play college or professional football, the two remained best friends and Lynch, who ran a restaurant in the Chicago area, frequently visited Alm in Houston and attended Oilers games.

"They were closer than most friends are," said Danny Schumacher, a loan officer in Chicago who was a receiver on the same team with Alm and Lynch.

"If you saw Jeff, you usually saw Sean with him," Schumacher said in a telephone interview, recalling that the two had taken several high school trips to Florida together and at one point even drove the same model car, a Lincoln Mark VII, with both good-naturedly competing over who had the more elaborate car stereo.

Alm's older brother, Lance, of Schererville, Ind., said in a telephone interview that the two men were "just extremely close, as close as brothers" and had taken a trip together to Las Vegas with their girlfriends over the summer.

"I think the whole accident was just so traumatic that Jeff lost his mind," he said. No Foreshadowing

The incident was a stunning development for the Oilers, who got off to a 1-4 start this season but have since been on an eight-game winning streak.

The head coach, Jack Pardee, said at a news conference at the Oilers' training facility that Alm had been "probably a little bit" depressed in recent weeks because of his frustration over having to sit out several games this season with an injury, a hairline fracture in his lower right leg.

But, Pardee said neither he nor anyone else on the team had reason to consider Alm suicidal, and Pardee speculated that he had been overcome with remorse over causing his friend's death.

"Evidently, he convicted himself for doing something wrong," the coach said of Alm. "It's a pretty stiff sentence he put on himself." An Impulsive Act?

Karyn Hall, a clinical psychologist in Houston and a board member of Crisis Intervention, a nonprofit agency dedicated to the prevention of suicide, said that while some people meticulously plan their suicides, there are cases in which a person who had not previously contemplated suicide acts suddenly, often in response to a traumatic event that strikes him or her as overwhelming.

"It's almost like emotional tunnel vision and they see no alternatives," Hall said. "And when there is access to weapons, it becomes a very impulsive act."

Hall said sudden suicides can also be triggered in part by alcohol or drugs, which lower the ability to control impulses. Police said they had no evidence of either in Alm's car, and a spokesman for the Houston Medical Examiners' Office said results of laboratory tests that would indicate whether either man had been drinking would not be available until later this week.

Asked whether Alm was taking any medication that could have helped provoke a sudden burst of rage, a team spokesman said he did not know what, if anything, had been prescribed for him in connection with his leg injury. Star at Notre Dame

The 6-foot-6-inch Alm was a star at Notre Dame, starting all 12 games in the team's 1988 national championship season. He was the Oilers' second-round draft pick in 1990 and last year he played in 14 games, collecting 35 tackles, 1 sack and 2 forced fumbles.

But he sat out nearly half this season in a contract dispute, and when he came back he played briefly in just two games as a backup tackle before sustaining a tibial plateau fracture in his right leg. He had talked about coming back against Pittsburgh this Sunday.

Pardee today described Alm as an "emotional guy with highs and lows" and "a very competitive player."

In Orland Park, Alm's former teachers and other school officials described themselves as stunned and shocked.

"We had a lot of pride in his accomplishments and so did the community," said Cliff Eade, the athletic director at Carl Sandburg High School who had known Alm since Alm was an eighth-grader.

"We're all walking around here shaking our heads, saying this can't happen," Eade added. "But it did. That's life. I wish they'd outlaw guns. If it wasn't there, I don't think that it would have been quite so easy for him to do what he did."

Jeffrey Alm