Philadelphia's prison population has been plunging for more than a year, as leaders from the Mayor's Office on down have sought alternatives to locking up people in overcrowded jails.

That success may have had its first unintended consequence this month, when the city ran out of the ankle bracelets necessary to send prisoners home with electronic monitoring.

City judges were warned that house arrest was no longer an option, as a waiting list of prisoners has grown during the last three weeks, court officials said.

Deputy Mayor Everett Gillison said last week that buying 100 more bracelets - for about $1,300 apiece - was a priority. He thought he could squeeze the money out of a lean budget by early this week.

"I'm cutting people and I'm cutting services, so I'm trying to figure out where I can get the money right now," he said. "It's going to take a couple days to figure out what I have to give up."

At the start of the week, there were 773 people on electronic monitoring and 73 on the waiting list, said David Lawrence, the court administrator.

"This is the first time we've encountered a waiting list," he said. "Historically, the numbers have been flat. We're just experiencing a bump right now."

Tom Innes, the director of prison services for the Defender Association of Philadelphia, said the 73 were "people in jail who could be out but for a lack of bracelets."

"We've been pushing for more bracelets for a while, but they're expensive," he said. "Everything has a price tag right now."

Usually, Innes said, there is a slight backlog of 10 or 15 people waiting for bracelets, but "73 is huge."

"I've never heard it that high," he said. "It's outrageous that it would be that high."

House arrest can be ordered as a punishment for a variety of convictions, depending on the severity of the crime, the defendant's criminal history, and the judge's discretion.

House arrest also can be used as a way to keep track of defendants awaiting trial. Pretrial house arrest is less common, but can be ordered for anyone eligible for bail.

Lawrence estimated that two-thirds of the waiting list consisted of post-conviction inmates.

Gillison and other city leaders could not cite a specific reason for the recent surge in house arrest.

Instead, they referred to a unified commitment among those in the city's law enforcement community to reducing the jail population.

Through a variety of measures, the population has fallen from more than 9,800 in January 2009 to about 8,100 last week, Gillison said. The drop has saved the city about $9 million.

"I've been a proponent of alternatives to incarceration throughout my legal career," he said. "I'm ecstatic that everyone across the board . . . is on the same page."

The need for more ankle bracelets was discussed last week at a meeting of the Criminal Justice Advisory Board, which includes police, judges, prosecutors, and defense attorneys, among others.

District Attorney Seth Williams showed the group a new bracelet that monitors whether the wearer has been drinking alcohol. A California judge ordered actress Lindsay Lohan to wear one last week.

"My thing is we need to incarcerate the right people," Williams said.