By fall 2011, months after taking office, Mayor Rahm Emanuel said he had fulfilled a campaign pledge to move some 1,000 cops to beat patrols to beef up the Police Department's front lines in the fight on crime.

But in the more than two years since then, those numbers have plunged by hundreds of cops, the city's own data show.

In spite of a renewed hiring push over the past two years, the department has struggled to keep up with retirements, in part because of sweetened medical benefits for police retirees as part of a cost-saving move a few years ago.

According to the data obtained through a Freedom of Information Act request, the department had 7,078 rank-and-file officers and supervisors assigned to work in its 22 patrol districts as of Dec. 8, a decline of 10 percent, or 779 beat officers, since fall 2011.

Emanuel and police Superintendent Garry McCarthy view the beat officers as the backbone of the department. McCarthy has stressed the need to put the necessary resources in the hands of his district commanders and then hold them accountable in the fight against crime.

"An officer tied to a radio, knowing a neighborhood, knowing its neighbors — that's how you affect crime," the mayor said in July 2011 as he announced that 40 officers assigned to desk jobs had been reassigned to beat patrols.

In addition to beat cops, there are many other types of cops, some also on the street. Close to 5,000 work as detectives, gang and narcotics specialists, on saturation teams and in other specialized units. The district tallies don't include them or about 515 rookie cops on foot patrol or in field training who have yet to be permanently assigned to districts. If those rookies were included in the district numbers, staffing would be down by about 3 percent since fall 2011.

But departmentwide hiring has not kept pace with retirements and other departures. According to a Tribune analysis of city data from several agencies, the Police Department has added a combined 1,495 cops since early 2009, but those gains have been swamped by the close to 2,600 officers who retired or left the department for other reasons.

According to city data, the department's overall staffing stood at about 12,250 as of the first of the year, down almost 900 officers from the end of 2009.

Under Emanuel, hiring has picked up, with 1,047 new cops hitting the streets over the past two years. Still, staffing hasn't gained. The department has 20 fewer cops than in December 2011, the data show.

With fewer cops on beat patrols, calls to 911 have routinely become backed up in some police districts, leading to longer response times, veteran district officers say.

Over the past five years, police districts on the North and Northwest sides took the biggest hits in manpower, while staffing grew in districts in higher-crime neighborhoods on the South and West sides, the data show.

Robert Lombardo, a criminologist at Loyola University Chicago, stressed the importance of maintaining adequate staffing throughout the districts, even those facing less violence and crime.

"It is not just about assigning (officers) to hot spots," said Lombardo, a Chicago cop for 30 years who retired as a sergeant. "There is the whole issue of (crime) prevention and having enough officers to properly work the beats."

McCarthy has long insisted that the department has the necessary manpower to do its job. He frequently notes that Chicago has the most officers per capita of the five biggest U.S. cities — that's supported by FBI statistics — and points to last year's sharp drop in homicides and shootings as evidence of the department's adequate staffing.

Many in the department, though, credit the turnaround in 2013 to the huge increase in overtime spending — about $100 million in all — that allowed hundreds of cops to work on their days off, mostly on the South and West sides.

Adam Collins, McCarthy's spokesman, did not dispute that district-level staffing has decreased, but he noted that those numbers are affected not only by retirements and other departures but also promotions of officers to other units. The department has kept up with attrition and has virtually no vacancies currently, he said.

"We have the number of officers we need," Collins said. When vacancies occur, he said, "we bring in more recruits in the (police) academy."

Almost 170 recruits are training at the police academy. More than 50 of them are to graduate Friday and then undergo three more months of field training before hitting the streets.

The Fraternal Order of Police, the union representing rank-and-file officers, and some aldermen have pushed to hire even more cops, but McCarthy defends overtime as the most cost-effective move and says he expects the overtime spending to ease in 2014.

Police hiring slowed in the late 2000s because of the recession and budget difficulties during the administration of Mayor Richard Daley.

In an attempt to cut costs, Daley and the police union agreed in 2009 to lower the age when officers with at least 20 years of experience could retire and still qualify for full medical benefits from 60 to 55. That led to a sharp increase in retirements beginning in 2010, saving the city millions of dollars, according to FOP acting President Bill Dougherty.

The union hoped those retirements would allow the city to hire new officers at lower salaries. Some were hired, but not enough to keep up with attrition.

"We're constantly playing catch-up," Dougherty said. "Even in years now when the department hires 400 officers, you're still replenishing what we lost years ago."

When Emanuel became mayor in May 2011, the department had 7,304 cops patrolling the districts, the city data show. By fall 2011, the numbers had risen to 7,857 at about the time Emanuel took credit for fulfilling his campaign pledge to add 1,000 cops. He did that by transferring scores of cops from desk jobs and disbanding two specialized units that responded to crime hot spots, scattering them into districts throughout the city. But critics said many of those specialized officers were already on the street, just not assigned to districts, meaning the mayor had simply shuffled existing resources without adding officers to the ranks. Indeed, the city's own numbers show staffing rose by about 550 officers in the districts.