A spokesman for the local office of the Federal Bureau of Investigation told The Washingon Post today that the bureau is investigating an alleged threat to fullback Jon Keyworth of the Denver Broncos.

Bronco coach Red Miller said of the incident: "I think it has been blown out of proportion. It's probably some kook trying to get a little publicity." But the FBI spokesman said later. "Under federal law, any time there is a threat to someone's life it is a major case."

He declined to elaborate, saying, "We have received a complaint. We are not furnishing a guard to Keyworth but we are maintaining contact with the National Football League security force, which is a good one, and with the Denver club. Jack Danahy, the head of NFL security, came to our office this morning."

Some Broncos remembered that a movie named "Superdome" was televised nationally Monday night. It involved gamblers betting $10 million on the underdog team in a game here and committing three murders in an attempt to make sure the favorite did not win.

"It was a terrible movie," NFL commissioner Pete Rozelle said today. "I must say I was deeply concerned that it was shown." He said it was "ill-timed" and "in poor taste".

"The power of suggestion in things like this is so great," he said, but declined to attribute the telephone threat to Keyworth to the showing of the TV film.

Fred Gehrke, general manager of the Broncos, said he was told by FBI office in Denver that the New Orleans office would send personnel to the Broncos' headquarters at the Sheraton Inn.

Gehrke said that during the night there has been an NFL security representative on each of the three floors where the players are quartered, but that a second man was assigned today to the floor where Keyworth is staying.

Keyworth did not appear with other players today for the usual interviews.

Gehrke reported he received a telephone call Wednesday from the chief of police in Denver. The chief said that a threat had been made of bodily harm to keyworth. "He told me I'd better beef up security here, but no to make a big thing of it because it was more or less rumor," Gehrke said. "I was told that Keyworth should not accept any mail.

"I told Jon and he said, 'I'd like to call my wife in Denver.' He was worried about their new baby. The FBI in Denver said she should be careful about opening her door to anyone. She is coming here today with other wives and families of our organization."

Gehrke remarked; "I wonder why it would be Keyworth (a sometime starter and not Morton (the quarterback)?

It was recalled that Joe Namath received a threat to his life about a week before this New York Jets upset the Baltimore Colts in the third Super-Bowl.

Placekicker Jim Turner of the Broncos, who played with the Jets in that game, said, "We get threats that you don't hear about. Namath used to get one almost every week."

Bronco linebacker Randy Gradishar did not discount the possibility of the threat being serious."I'd be upset if my wife were threatened," he said.

Gehrke said that when he first told Miller of the news, the coach said, "Let's be careful: let's not let the rest of the team know."

But, Gehrke noted, "It got out, in the newspapers and on television."

He said Keyworth operates a restaurant and bar in the Denver area called "The Chaparrak." Keyworth also had recorded a song entitled, "Make Miracles Happen," and Gehrke said it became widely popular around Denver.

The 6-foot-3, 230-pound native of San Diego was drafted by the Washington Redskins out of the University of Colorado and was traded to Denver in his rookie season, 1974.

Instead of coach Miller opening the daily news conference today, Gehrke appeared at the microphone and said, "There has been a threat on Keyworth's life. He won't be available here (for interviews). He's a little bit shook. We got a call from Denver that has been threatened with bodily harm. That's all we know."

Then Miller answered nongermane questions until the subject of the threat was raised and he suggested the incident was being overemphasized.

Asked if there had been other threats this season, he said. "None that I know of; none that surfaced."

In reply to a question about Keyworth practicing, he said. "He won't be limited" and justed, "We might have Claudi Minor and Paul Howard (offensive linemen) on either side of him for a while."