Jerry Larson was trying to get some sleep Monday night when his 27-year-old son, Aaron, sat on the edge of the bed, asking for advice.

Jerry Larson said he shooed his son away, telling him to go enjoy the family's Christmas Eve turkey dinner. He promised they would talk after he had a few hours of sleep, but by the time Larson awoke his son was gone.

The next day, Aaron Larson was shot to death by a State Patrol trooper during a bizarre incident on Interstate 5 in Federal Way.

Several people called 911 around 2:30 p.m. Tuesday to report that a man had jumped out of a moving car near the onramp at South 320th Street. Witnesses said he dropped his pants around his ankles and was exposing himself, said Stacy Flores, spokeswoman for the Federal Way Police Department, which is investigating the shooting.

By the time troopers arrived, the man had his pants back on but he had his belt in his hand and was using it to strike passing cars, Flores said. He also tried to pry open some car doors.

The man tried to choke Trooper Mike Cheek and struck him repeatedly with his hands and fists, Flores said. Cheek used a Taser on Larson but it was ineffective, she added.

Cheek then shot the man. While the King County Medical Examiner's Office hasn't released the man's name, his father identified him as Aaron Larson.

Cheek was not hurt and has been placed on administrative leave, the State Patrol said. Cheek has been with the State Patrol for 11 years and has not been involved in another fatal shooting while with the department.

Authorities are investigating whether Larson was under the influence of drugs. The King County Medical Examiner's Office said Wednesday it is awaiting toxicology tests.

"His behavior was unusual, not normal," Flores said.

At least one 911 call was made the car in which Larson was in, with the caller indicating the man had a handgun, police said. Larson's girlfriend, her son and Larson's younger brother were all riding in the car when he hopped out, according to Jerry Larson.

Detectives are investigating whether Larson was armed. It's unknown whether he drew a weapon at any point during Tuesday's incident, Flores said. Jerry Larson said Wednesday he doesn't believe his son was using drugs because he was working for a Chehalis-based trucking company that conducts random drug tests on its drivers.

Stephanie Axelson, secretarial payroll director at M&M Transport, said Larson tested negative for drugs when he was hired three months ago. She said he hadn't been tested since.

Jerry Larson, 50, of Ethel, Lewis County, said his son had recently been depressed because things weren't going well with his girlfriend and that he didn't like living in Olympia. Aaron Larson grew up near Snoqualmie and graduated from Mount Si High School, his father said. He moved to Honolulu after high school, but recently returned to the region, settling in Olympia with his girlfriend and the woman's young son.

Axelson said Larson often talked about his family life and bragged about being "a dad."

"He was really a nice guy. He wasn't a loud personality at all," Axelson said. "This has totally got the people who knew him the best completely dumbfounded. This was not the Aaron we knew."

Jerry Larson said his son had been known to be confrontational and would never back down from a fight. While he said he hasn't talked to anyone with the State Patrol, he thinks troopers could have calmed his son down instead of shooting him.

"Any man that gets out there on the freeway with a belt buckle in his hand, what's he going to do, dent a few cars?" he said. "I just think they gunned him down."

Jennifer Sullivan: 206-464-8294 or jensullivan@seattletimes.com