School teacher David Draa shot and killed his friend and colleague as he knelt over to fix a flat bicycle tire before he took his own life, police say.

The murder-suicide Sunday in Montgomery County has puzzled detectives who say the victim, Steven "Kyle" Hickman, 30, had been trying to help Draa, also, 30, through personal issues, possibly from fighting in the Iraq War, police said.

The motive remains unclear, said Takoma Park Police Chief Alan Goldberg.

"We can only speculate as to why this occurred," Goldberg said in a statement. "We believe that David had experienced severe trauma during his tour of duty while serving in the Marine Corps, as well as some difficulties in his personal life."

According the Department of Defense, Draa served as an infantry officer in Iraq in 2006.

Draa eventually moved back to his home state of Maryland, and he and Hickman became friends and co-workers at High Road Primary and Middle Academy, a special education school in the District. Draa was named staff member of the month at the school in December.

"Mr. Draa is an amazing teacher," the school website says. "He is very patient and kind to all he encounters ... Mr. Draa is able to help de-escalate a student that is in extreme crisis to the point they even forgot why they were mad in the first place."

Police said the two men went biking to Wheaton on Saturday, and Hickman had a flat tire on the ride back.

Hickman, of D.C., texted his wife that evening, saying he was going to stay at Draa's apartment in Takoma Park, and fix the tire the next morning.

The men drank alcohol and met Draa's neighbors, police said. The neighbors said they did not see any hint of a problem between the two friends, police said.

About 1:30 a.m. Sunday, Hickman changed his mind and decided to fix the tire and possibly ride home. Police don't know why Hickman changed his mind, and there was no evidence of an argument or physical confrontation.

As Kyle knelt over his bike in the driveway, David shot his friend from behind, police said.

"We believe that Kyle never knew what happened," Goldberg said.

Draa knelt over his friend's body, then took his own life, police said.