The FBI won't take over an investigation into the homicide of a Baltimore city detective who was shot in the head the day before he was to testify before a federal grand jury about a group of indicted officers, the city's top police official said Wednesday.

FBI officials have seen no evidence to suggest Detective Sean Suiter's death was "directly connected" to the federal corruption investigation, therefore FBI officials believed it would be "prudent" for the police to continue leading the inquiry, Baltimore Police Commissioner Kevin Davis said, quoting a letter he said he received from FBI Assistant Director Stephen Richardson.

"That means that they believe that at this moment there's no conspiracy to murder Sean Suiter by someone else involved in that ongoing case," Davis told reporters.

Rumors about Suiter's Nov. 15 shooting went into overdrive last month when Davis confirmed that the acting U.S. attorney and FBI informed him the detective was gunned down the day before he was to testify in an ongoing federal investigation of a disbanded gun-recovery unit. Eight Baltimore officers have been indicted on charges they defrauded their department, falsified evidence and shook down citizens. The same week Suiter was shot, a Philadelphia officer became the ninth officer indicted in the investigation of Baltimore's Gun Trace Task Force.

Suiter was attacked while investigating a 2016 triple homicide with his partner in a high-crime neighborhood. Davis said Suiter approached a "suspicious" man in a vacant lot between row houses, leading to a violent confrontation in which he was shot with his own gun. His partner can be seen on private surveillance video taking cover across the street, according to Davis.

For weeks, Davis has repeatedly said his department had no reason to believe Suiter's death was connected to his pending testimony, but he also has stressed that investigators are not ruling out anything.

He sent a formal request on Dec. 1 asking the FBI to take over the investigation. In his request to the FBI director, Davis said the investigation was "significantly complicated by the fact that he was to appear before a grand jury the following day."

On Wednesday, he told reporters that he took the "unusual step" of asking the FBI to lead the investigation "primarily because of the extraordinary circumstances associated with the death" of Suiter and because he knows that many people "think there is corruption involved in this."

"It was out of an abundance of caution. I wanted the credibility to be at its very highest level," said Davis, adding that he has full confidence in his department and Baltimore has "the best detectives" anywhere.

There have been no arrests, despite a $215,000 reward.

Davis said there is "zero evidence" to support some speculation that the homicide detective staged a suicide. And he stressed that the FBI does not think that Suiter's death "has anything to do with that pending testimony."

"If they did I have no doubt in my mind that the FBI would have taken this case," he said, adding that "what we have left before us is a murder committed by a yet-to-be-unidentified perpetrator."

He emphasized that the Suiter investigation was an active investigation.

FBI agents and other federal authorities have been assisting Baltimore police in the high-profile homicide investigation for weeks.

Suiter's widow, Nicole, also wanted the FBI to lead the investigation into her husband's death, according to Davis and Baltimore Mayor Catherine Pugh. Davis said she was aware of the FBI's decision.

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David McFadden on Twitter: https://twitter.com/dmcfadd