President Obama listens to remarks while hosting a community discussion on drug addiction during a visit to Charleston, W.Va., on Oct. 21, 2015. (Kevin Lamarque/Reuters)

President Obama said Tuesday that there is no evidence police officers in major cities have pulled back from enforcing the law out of concern they will come under fire for using harsh tactics.

Twice in the past week, FBI Director James B. Comey suggested that exactly such a phenomenon was responsible for rising crime rates in many U.S. cities, as police officers worry that their behavior will be captured on video and go viral. “A chill wind has blown through law enforcement over the last year, and that wind is surely changing behavior,” Comey said Friday.

But speaking here Tuesday to the International Association of Chiefs of Police, Obama said, “It is true that in some cities, including here in my home town of Chicago, gun violence and homicides have spiked — and in some cases they’ve spiked significantly. But the fact is that so far at least across the nation, the data shows that we are still enjoying historically low rates of violent crime.

“What we can’t do is cherry-pick data or use anecdotal evidence to drive policy or to feed political agendas,” he said.

Comey acknowledged that he had seen only anecdotal evidence that some cities are experiencing what some have called the “Ferguson effect,” a reference to last year’s police shooting of black teenager Michael Brown in Ferguson, Mo., which set off a national crusade against police brutality.

At least 30 of the nation’s largest cities had seen a year-over-year increase in homicides as of Sept. 30, according to data kept by the Major City Chiefs Association.

In Washington, D.C., Police Chief Cathy L. Lanier has said the city hasn’t seen a homicide wave like the current one in “decades.” In St. Louis, were police officials have said they’re confounded by how to dampen the homicide rate, the city had seen its 159th slaying of the year by Oct. 15, matching last year’s total with more than two months of this year remaining.

But many civil rights groups and criminal justice advocates have been quick to note that 2014 was a year of historically low homicide rates. They say the year-over-year increases do not signal a crime wave.

Jeffery Robinson, director of the American Civil Liberties Union’s Center for Justice, said there was not enough data to make a direct connection between increased scutiny of police and current crime rates.

“This line about the ‘Ferguson effect,’ it might ring a nice political tone, but the facts just don’t back it up,” he said. “The message seems to be: ‘Shut up complaining about the unnecessary use of force in your community, or the police will stop protecting you.’ ”

A report released Tuesday by the Justice Department said that ambush attacks against law enforcement officers remain a threat to officer safety across the country, with the number of attacks per year holding steady at about 200 a year.

The study by the department’s Office of Community Policing examined the ambushes — or planned surprise attacks — on law enforcement officers between 1990 to 2013 and concluded that concerns about targeted violence against police were on the rise and that “officers must not only be guardians of the public but also be prepared to respond to violence targeting them.”

“This report will serve as a critical base of knowledge as we work to defend our law enforcement and ensure our officers’ safety,” Attorney General Loretta E. Lynch said in a statement. Lynch was scheduled to travel to Chicago on Tuesday to address the police chiefs association but had to cancel her trip because she was ill.

Obama, who also headlined two Democratic Party fundraisers and attended the Chicago Bulls’ season-opening game against the Cleveland Cavaliers during his trip, continued his push for criminal justice reform in his talk to police chiefs.

“So this is something I don’t get to say very often: I am encouraged by what Congress is doing,” he said.

A bipartisan measure that proposes to change federal sentencing and prison rules passed the Senate Judiciary Committee 15 to 5 on Thursday, and House Judiciary Committee Chairman Bob Goodlatte (R-Va.) has drafted a bill with some similar sentencing provisions.

Sen. Richard J. Durbin (D-Ill.), one of the authors of the Senate compromise, said that when he saw Obama at lunch Monday, the president indicated that he is reaching out to both sides to determine where he can be helpful.

“He wants to move in ways that keep Republican support strong,” Durbin said in an interview. “We don’t need a backlash on this.”

On Capitol Hill, there is still a heated debate over the sentencing reform proposals. Reps. F. James Sensenbrenner Jr. (R-Wis.) and Robert C. “Bobby” Scott (D-Va.) introduced a comprehensive measure in June that would impose mandatory minimum sentences on higher-level drug traffickers rather than low-level offenders, apply life sentences for drug trafficking only in extreme cases, and allow eligible offenders to petition for resentencing under new trafficking laws.

In testimony last week before the Senate Judiciary Committee, Patti Saris, head of the U.S. Sentencing Commission, said that the measure’s lower mandatory minimums for prior drung felons would benefit about 84 offenders annually and that as many as 2,265 offenders could be released early over a period of several years. She did not estimate how many people would be subject to new mandatory minimums because of a provision applying these sentences to those with prior “serious violent offenses.”

Durbin said the original bill he and others drafted would have released more low-level drug offenders. “But we had to give ground to get Grassley’s support,” he said, referring to Sen. Charles E. Grassley (R-Iowa). “I learned a while ago that if the chairman of the Senate Judiciary Committee is the lead sponsor, it helps.”

Durbin added that the new mandatory minimums would be triggered by a small number of violent crimes and would not have a “significant” impact on how many people remain in prison for extended periods.

Hilary Shelton, director of the NAACP’s Washington bureau, said in an interview Tuesday that while his group has endorsed the Senate bill, it has “deep concerns” about its expansion of mandatory minimums and would like to see legislation that addresses the issue of excessive use of force by police in some instances.

“We’re still not talking about policing issues,” Shelton said.

Obama praised the Senate measure Tuesday, saying, “I hope they get a bill to my desk so that I can sign it, and together we can work to keep reducing America’s crime rate and its incarceration rate at the same time.” The Justice Department has provided technical assistance for some proposals, according to several lawmakers and aides, but the White House has not actively lobbied on them.

It is unclear how quickly the Senate Judiciary measure — which won the support of all of the panel’s Democrats but was opposed by Sens. Ted Cruz (R-Tex.), Orrin G. Hatch (R-Utah), David Perdue (R-Ga.), Jeff Sessions (R-Ala.) and David Vitter (R-La.) — will advance. Don Stewart, spokesman for Senate Majority Leader Mitch McConnell (R-Ky.), said in an e-mail that it is unlikely to make it onto this year’s floor schedule, given pending bills on funding the federal government and raising the debt ceiling.

And while groups from across the political spectrum have endorsed the bill — including the Leadership Conference on Civil and Human Rights, the ACLU, Koch Industries, FreedomWorks and the American Bar Association — some law enforcement organizations have raised concerns.

The National District Attorneys Association, for example, questions whether the early-
release provisions could “lead to reopening old wounds for victims,” and the Major County Sheriffs’ Association says they could impose a “burden borne by local communities.”