When Micah Johnson opened fire on Dallas police in an act of vengeance against white officers, he was attacking a department whose chief has been lauded across the country for taking bold steps to root out bad cops and repair relations with minorities.

Police Chief David Brown, a black man who pushed through the reforms despite resistance from the rank-and-file, boasted at a news conference on Monday that crime, police shootings and excessive-force complaints against the department have all dropped dramatically on his watch.

"This is the best department in the country, and I'm proud to be associated with the men and women of the Dallas Police Department," he said.

Johnson, a black Army veteran who served in Afghanistan, killed five officers in a sniper attack on Thursday that he portrayed as payback for the fatal police shootings of black men last week in Baton Rouge, Louisiana, and suburban Minneapolis. The attack ended with Johnson blown up by a bomb delivered by a police robot.

No evidence has come to light to suggest that the 25-year-old Johnson had a grudge specifically against the 3,400-officer Dallas Police Department.

"Dallas PD is paying the price for problems elsewhere around our country," said Mohamed Elibiary, a Texas-based Department of Homeland Security adviser.

Carlyle Holder, president of the National Association of Blacks in Criminal Justice, had been holding up the Dallas Police Department as an example of a law enforcement agency effectively addressing the problem of racial disparities in police work.

"That's what made the killing of those officers so much harder to take," he said.

Brown became chief in 2010, taking over a department that a generation ago had one of the highest rates of civilian shootings in the country. In 2012, his efforts to heal the rift between police and the black community took on greater urgency when the killing of a black man by a white officer triggered widespread protests.

The chief responded by creating a public database to track shootings by police, requiring officers to undergo lethal-force training every two months instead of every two years, and firing 70 officers involved in questionable incidents, including some who faced charges of excessive use of force.

The biggest backlash has come in the last six months, when Brown started a community policing program in mostly Hispanic and black neighborhoods. He began reassigning officers from desk jobs to foot patrols, a move that was praised by criminal justice experts but angered the police unions, who demanded his resignation.

"We need more boots on the ground, absolutely," said Mike Mata, vice president of the Dallas Police Association. "The problem is he was reassigning detectives from the office to back on the streets without any retraining. It's almost like you set them up for failure."

The dispute further strained a department that Brown said is one of the lowest-paid in the region, with new recruits making $43,000 a year, and has had such trouble recruiting officers that academy classes are frequently canceled for lack of participation.

While attending the University of Texas at Austin on a full scholarship, Brown became motivated to enter law enforcement when he returned to Dallas in the early 1980s and found friends from the neighborhood caught up in a cocaine epidemic. Months after he became chief, his 27-year-old son, David Brown Jr, was shot to death by police after killing two officers.

"I've been black a long time," Brown told reporters Monday. "We're in a much better place than when I was young man here, but we have more work to do, particularly in my profession."