Young said that he hoped the mayor would choose a new commissioner from the department, but he declined to name possible successors. Among the most experienced in the department are deputy commissioners Anthony Barksdale and John Skinner, and Col. Jesse Oden.

Mayor Sheila Dixon chose Bealefeld to head the department shortly before the 2007 mayoral election. At the time, the city's murder rate was climbing to the highest levels in years and arrest numbers were soaring, sparking complaints from judges and civil liberties groups.

Dixon said she chose Bealefeld over another top contender — a former District of Columbia police chief — because his passion and personality won her over.

"There was something about his spirit and the connection that we made that really helped me determine he could implement that whole three-pronged approach to fighting crime," Dixon said. "It was one of the best decisions that I made. He was an extraordinary person and he really made history."

Politicians who represent the city in Annapolis expressed both surprise and admiration. House Majority Whip Talmadge Branch, who represents East Baltimore, noted the drops in crime, as well as the commanders Bealefeld chose for the districts.

Branch endorsed the idea of a national search for a successor, adding that the department could use "some new people and some new ideas."

Sen. Verna Jones-Rodwell of West Baltimore called the commissioner's departure "an opportunity to look outside of Baltimore. But the candidate has to be [one] with a strong track record with the urban setting."

Del. Keiffer J. Mitchell Jr., who also represents West Baltimore, said Bealefeld would be missed. "He was one I've always had a good relationship with, even when he was a major and I was on the City Council. He was one of those rare commissioners that wasn't political."

The commissioner appeared re-energized two years ago when Gregg L. Bernstein became the city's new top prosecutor, unseating an incumbent with whom Bealefeld and several predecessors had openly fought, claiming she undermined police efforts to fight crime.

The Bealefeld-Bernstein duo signaled a new cooperative spirit billed as a potent crime-fighting front, and on Thursday Bernstein said that his friend will go down as "one of the best police commissioners" the city has ever had.

"I think through sheer dint of will and effort, he greatly improved the effectiveness of the Baltimore Police Department, and I think that the statistics bear that out in terms of the drop in crime," Bernstein said. "And I think he deserves a large amount of the credit for that. I think that the citizens of Baltimore owe him a great debt of gratitude and respect."

When Bernstein ran for office in 2010, Bealefeld endorsed him by placing a sign in the front yard of his home. The commissioner took heat for putting himself and his office in the middle of a political campaign, but that support was credited with giving the upstart Bernstein needed publicity.

Earlier this year, Bernstein and Bealefeld launched a community prosecution model that put prosecutors in charge of specific geographic zones. It connects them to police in those zones, in hopes that the two agencies will become more closely aligned.

Bernstein declined to describe what he would hope for in a successor, saying that was up to Rawlings-Blake.

Bealefeld joined the Baltimore Police Department as a cadet in 1981, dropping out of Anne Arundel Community College when a broken collarbone ruined his chances for a lacrosse scholarship. He took his academy entrance exam wearing a neck brace.

Bealefeld moved up through the ranks and served in a variety of roles — including patrol, homicide, narcotics, and as commander of the Southern District station. He followed in the footsteps of his grandfather, who once walked a beat along Greenmount Avenue, and a great-uncle, who was killed in the line of duty.

Maryland State Police Superintendent Marcus Brown served with Bealefeld for many years in the city, and commanded the Northwestern District when Bealefeld headed the Southern.

"The one thing that has always stood out about him is his commitment to the citizens of Baltimore," Brown said. "You never had to question where Fred was on any issue. His integrity was a staple for him from the time he was an officer until Aug. 1 when he retires."

Brown, who left the city force just as Bealefeld took over as commissioner, said their discussions almost always involved "his struggle with the crime fight. … He never talked about the job wearing on him to the point where he wanted to retire. All of our conversations were about what's the next step toward making the city safe."

In his interview last year, Bealefeld discounted speculation that he was about to depart.