The acting director of a Baghdad psychiatric hospital has been arrested on suspicion of supplying Al Qaeda in Iraq with the mentally impaired women it used to blow up two crowded animal markets in the city on Feb. 1, killing about 100 people.

Iraqi security forces and U.S. soldiers arrested the man at al-Rashad hospital in east Baghdad on Sunday. They then spent three hours searching his office and removing records. Sources told The Times that the two female bombers had been treated at the hospital in the past.

"They [the security forces] arrested the acting director, accusing him of working with Al Qaeda and recruiting mentally ill women and using them in suicide bombing operations," a hospital official said.

Ibrahim Muhammad Agel, director of the hospital, was killed in the Mansour district of Baghdad on Dec. 11 by gunmen on motorbikes. Colleagues suspect he was shot for refusing to cooperate with Al Qaeda.

Even before Sunday’s arrest, U.S. officials believed that Al Qaeda was scouring Iraq’s hospitals for mentally impaired patients whom it could dupe into acting as homicide bombers. They said that Al Qaeda had used the mentally impaired as unwitting bombers before.

"We have fairly good reason to believe this is not the first time they have recruited mentally handicapped individuals," said one senior officer, though he did not think there had been more than half a dozen cases.

The attraction of mentally impaired women to Al Qaeda was obvious, he said. Being women they could get close to targets with less chance of being stopped or searched; being mentally impaired, they were "less likely to make a rational judgment about what they are being asked to do."

The Feb. 1 attacks were the deadliest — and most chilling — to hit the Iraqi capital in months. One of the women was given a backpack full of explosives and ballbearings, the other a suicide vest laden with explosives. They were sent into the middle of al-Ghazl and New Baghdad markets, which were packed with people. Their explosives were then detonated by remote control.

The Times was shown photographs of the two young women’s severed heads, which were recovered from the wreckage. One very obviously had Down syndrome. The other had the round face, high forehead and other features often associated with Down syndrome, but her symptoms were less pronounced.

An insight into the way Al Qaeda thinks came in a letter written by one of its leaders in Anbar province that the U.S. military seized in November and released in part on Sunday. "It is possible to use doctors working in private hospitals and where the infidels/apostates are treated who have serious conditions to be injected with [air bubbles] that will kill them," it said.

The U.S. military believes Al Qaeda is adopting these extreme tactics because the prevalence of checkpoints and concrete barriers is making car bombings harder, and fewer foreign bombers are reaching Iraq.

The number of car bombs has fallen steadily from a peak of 112 last March to 27 last month. Conversely, there were 16 pedestrian homicide bombs in January, the second-highest total in 13 months.

Foreign jihadists — invariably male — used to carry out 90 percent of the homicide bombings in Iraq, but the U.S. military believes that tighter controls have halved the influx to 50 or 60 a month.

The officer conceded that protecting public places against individual bombers was almost impossible. "You really can’t stop a determined bomber from blowing themselves up," he said. "The key is continuing to take down the terrorist network that conducts these operations."