City officials are still mum on why Shaun O. Harrison, a reverend and Boston Public School employee accused of shooting a 17-year-old student execution-style Tuesday night, was allowed to work around youths despite what prosecutors say was apparent drug and gang activity at his home.

“Certainly this situation is very troubling to me,” Mayor Marty Walsh said yesterday. “We’re talking about some of the most vulnerable kids in our system, in our high schools, and somebody who’s supposed to be a mentor to them.”

He said an investigation is ongoing, and there will be “more to say once the investigation is complete.”

Harrison, 55, of Roxbury, an English High School “dean of academy,” was charged Thursday with assault with intent to murder, and police say more charges are expected after detectives searched his Pompeii Street home Friday night and found two handguns, a rifle, a shotgun, ammo, “trafficking weight” of cocaine and a large amount of marijuana.

“Additional charges are expected but haven’t been finalized yet,” Suffolk District Attorney’s Office spokesman Jake Wark said.

A neighbor in court papers said he had seen drug activity at Harrison’s apartment and reported it.

“I’ve alerted the police and City Hall on many occasions about the constant drug activity,” the tenant told police according to a Boston Municipal Court criminal complaint.

But police yesterday said they never received suspicious reports about the disgraced reverend.

Police Commissioner William B. Evans said that officers were sent to investigate drug activity at “nearby addresses,” but never to Harrison’s apartment.

“We received two complaints nearby in the late spring and early summer, and both were vetted out by our drug unit. We investigated all the complaints and believe they were unfounded,” Evans said. “They were for other addresses on Pompeii, but not his.”

Harrison was fired from his job at English High two days after the shooting, and officials have not been forthcoming about whether there had been any flags raised over his conduct in the schools.

On Thursday, parents of students at the Jamaica Plain high school received two phone messages from the principal, Ligia Noriega — one announcing what had happened, and a later call with information about counseling and teacher support available for students.