But some here suggested that the president’s efforts were unlikely to yield significant results. Representative Danny K. Davis, Democrat of Illinois, said Mr. Trump should be increasing funding for social service programs, and cautioned, “I don’t think we’re going to be able to police ourselves out of the violence.”

“The way you’d really do that is change people’s attitudes, change their minds, change the approach to life,” said Mr. Davis, whose teenage grandson was shot and killed in Chicago last year. “I don’t think the A.T.F. will do much in terms of the violence or the crime.”

Shari Runner, the president of the Chicago Urban League, said the A.T.F. collaboration could be helpful, even though she said she had doubts about the president’s motives.

If it “reduces the amount of guns flowing into the city, I’m all for it; let’s do it,” Ms. Runner said. “But it’s not because the president all of a sudden decided he was going to send the feds in. That’s a lot of hype and hyperbole.”

In February, the A.T.F. announced it would send more agents to Chicago. This week, police officials said the bureau was stationing a mobile ballistics lab in the city for part of the summer, when violence here traditionally peaks. But the practical effect remains unclear.

“Six months ago, we made it clear that we would welcome additional federal support, and six months later, we appreciate the 20 new A.T.F. agents that are now arriving,” Mr. Collins said. “But the progress C.P.D. has made this year has happened without any of the new resources from the federal government we requested.”

Through Friday, 1,360 shootings had been reported in the city, about 200 fewer than during the same period in 2016. The number of criminal homicides — 320 so far in 2017, compared with 322 at this point last year — has hardly changed.