SEATTLE (AP) — Few clues have emerged from a drive-by shooting that left a nine-year veteran Seattle police officer dead and a trainee injured on Halloween night.

Investigators on Sunday fielded tips, interviewed potential witnesses and scoured through video tapes for evidence that would lead them to the assailant or assailants. They have not yet identified a suspect or the suspect's vehicle, nor have they determined what kind of weapon was used.

Killed in the Saturday night shooting was field training officer Timothy Brenton, 39. Rookie officer Britt Sweeney, 33, suffered a minor injury.

The two were discussing a routine traffic stop while parked in downtown Seattle when their police cruiser was suddenly struck several times by gunfire shortly after 10 p.m., officials said.

Assistant Police Chief Jim Pugel said Brenton died instantly. Sweeney ducked, and a bullet grazed her back. She then called for help and returned fire, Pugel said.

"This is an assassination, and every resource is being used to bring it to a conclusion," Pugel said.

Brenton became the city's first officer killed in the line of duty since 2006, and Saturday's shooting was the first intentional homicide of an officer since 1994, police officials said.

Pugel said there had been no threats against Brenton and no indication the assailants were looking for him.