Baghdad, Iraq ? The number of prisoners held in U.S. military detention centers in Iraq has risen without interruption since autumn, filling the centers to capacity and prompting commanders to embark on an unanticipated prison expansion plan.

As U.S. and Iraqi forces battle an entrenched insurgency, the detainee population surpassed 11,350 last week, nearly a 20 percent jump since Iraq’s Jan. 30 elections. U.S. prisons now contain more than twice the number of people they did in early October, when aggressive raids began in a stepped-up effort to crush the insurgency before January’s vote.

Anticipating continued growth in the detainee population, U.S. commanders have decided to expand three existing facilities and open a fourth, at a total cost of about $50 million.

The steady influx of prisoners has also required additional U.S. military police officers to guard the detention centers. Commanders had hoped to use the MPs to help train Iraqi police, but management of the detention centers has taken priority.

“We’ve got a normal capacity and a surge capacity,” said Maj. Gen. William H. Brandenburg, who oversees U.S. military detention operations in Iraq. “We’re operating at surge capacity.”

Last month at Abu Ghraib prison, on the outskirts of Baghdad, the detainee population had grown so large that U.S. authorities decided to stop accepting new arrivals for a few days, Brandenburg said. Instead, detainees were held longer at field camps before being moved to Abu Ghraib.

The large number of detainees and uncertainty about their fates have become a political issue, with representatives of Iraq’s Sunni Arab minority demanding that the inmates be tried quickly or released. More than three-fourths are Sunnis, a fact that officers here say reflects the dominant role Sunni groups have played in the insurgency.

Brandenburg said he has argued for allowing the cases work their way through a process that includes a review board staffed by six Iraqis and three members of the U.S.-led multinational force. As of last week, he said, the board had looked at 10,000 cases and approved about 6,000 releases.

But Brandenburg acknowledged that the prisons were filling up faster than cases could be reviewed. “We’re still getting more detainees in than we’re getting rid of,” he said.

A second review board is being established this week to relieve some of the strain on the reviewers, who are facing a heavier work load. Together, the two boards should be able to handle 650 to 700 reviews a week, Brandenburg said. Iraq’s Central Criminal Court, created a year ago, has also picked up its pace. It handled 87 trials and 50 pretrial investigative hearings in March.