Baghdad ? Dozens of U.S. Humvees and Bradley fighting vehicles took up positions around Sadr City at nightfall Wednesday, as American forces pressed the search for five Britons kidnapped in a mock police raid that Iraqi officials said was carried out by the Mahdi Army Shiite militia.

A secret incident report about the abductions – written by Najwa Fatih-Allah, director general of the Finance Ministry’s data processing center, where the Britons were seized – quotes Gen. David Petraeus, the U.S. commander in Iraq, as saying the Mahdi Army “will be profoundly sorry” if it carried out the assault.

Much of the Mahdi Army militia is said to be loyal to radical Shiite cleric Muqtada al-Sadr, who resurfaced last week after nearly four months in hiding, apparently in Iran, and demanded U.S. troops leave Iraq.

Al-Sadr’s return appeared to be partly an effort to regain control over his militia, which had begun fragmenting. It was unclear whether the 33-year-old cleric would have been aware of or condoned the kidnapping of the five British citizens – four bodyguards and an employee of a management consulting firm.

When al-Sadr went underground at the start of the U.S.-led security crackdown on Baghdad 15 weeks ago, he ordered his militia off the streets to prevent conflict with American forces. Nevertheless, his return likely complicates U.S. efforts to crack down on violence and broker political compromise in the country.

Foreign Minister Hoshyar Zebari told BBC radio that the government was “vigorously” working to find the attackers but acknowledged the government has long believed that militia members have infiltrated its security forces.

“The number of people who were involved in the operation – to seal off the building, to set road blocks, to get into the building with such confidence – (means they) must have some connection,” he said.

Residents of Sadr City said hundreds of American and Iraqi troops sealed off areas of the Shiite neighborhood overnight and carried out arrest raids that lasted until dawn. The residents spoke on condition of anonymity out of fear of reprisals.

The U.S. military said in a statement Wednesday that it had arrested five suspected militants and one suspected leader of a militant cell during early morning raids in Sadr City. Those arrested were believed part of a cell that smuggled weapons from Iran and sent militants to Iran for training, the statement said.

The statement did not link the raid to the missing men.