Story highlights Detective Sean Suiter was shot a day before he was to testify before a grand jury

The reward for the arrest and conviction of the officer's killer is $215,000

(CNN) Baltimore police officials have asked the FBI to take over the investigation into the fatal shooting of a homicide detective the day before he was set to testify in a police corruption case involving fellow officers.

Sean Suiter, 43, was shot in the head with his own gun after struggling with his killer November 15 in west Baltimore. Suiter died the next day at a hospital. The 18-year department veteran had been investigating a killing when he noticed a man acting suspiciously before the shooting.

Baltimore Police Commissioner Kevin Davis said the FBI and federal prosecutors have said Suiter was not a target of the investigation into several Baltimore officers with a gun task force who were federally indicted in March . There is no apparent connection between Suiter's killing and the corruption case, the commissioner said.

In the December 1 letter to FBI Director Christopher Wray, Davis said, "the circumstances surrounding Detective Suiter's killing are significantly complicated" by the fact he was set to give grand jury testimony the day after he was killed.

"I am growing increasingly uncomfortable that my homicide detectives do not know all of facts known to the FBI or (United States Attorney's Office) that could, if revealed to us assist in furthering this murder investigation," Davis wrote.

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