Iraq's government opened a probe Monday into a British-Iraqi raid on a police intelligence headquarters in southern Iraq that captured an alleged death squad leader and found 30 prisoners with signs of torture.

The raid took place Sunday at the National Iraqi Intelligence Agency building in Basra, Iraq's second-largest city, 340 miles southeast of Baghdad.

Inside, troops discovered 30 prisoners with signs of torture and abuse, including one woman and two children, the British military said in a statement. It did not elaborate.

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An alleged death squad leader was captured along with four other suspected militiamen, Maj. David Gell, a British military spokesman, said Monday.

"They were suspected of serious criminal activity, including kidnap, torture, murder and involvement in roadside bomb attacks on multinational forces and civilians," Gell said.

The prisoners were not intentionally released but escaped after the operation, the British military said.

Iraqi Prime Minister Nouri al-Maliki ordered an investigation into the raid and vowed to punish "those who carried out this illegal and irresponsible act," his office said in a statement late Sunday.

Government officials were in Basra on Monday to begin an investigation, a spokesman said.

"We have a committee that is in Basra now and it has started work to uncover the circumstances of the incident," said Ali al-Dabbagh, a government spokesman.

More than 200 British troops were involved in the raid, along with an unknown number of Iraqi forces, Gell said.

British and Iraqi forces stormed the building without warning, and "the method of entry ... was appropriately robust," the statement said.

It was unclear whether the suspects worked at the intelligence agency or had taken refuge there.

Sunday's raid came a day after Iraqi commandos arrested a suspected militia leader, from whom they gleaned information that enabled them to carry out the operation, the statement said.

In December, British forces raided another police station in Basra after receiving intelligence that a renegade Iraqi police unit might execute its prisoners. Leaders of the station's serious crimes unit were suspected of involvement with local death squads, the British military said at the time.

In that raid, British troops transferred all 76 prisoners to another facility, and then destroyed the station with explosives, the military said.

Britain has 7,100 troops in Iraq, mostly based around Basra. Prime Minister Tony Blair said last month he would withdraw about 1,600 troops from Iraq over the coming months, and bring the number of troops to below 5,000 by late summer — if Iraqi forces can secure the southern part of the country.

More than 125 British personnel have died in Iraq since the U.S.-led invasion in March 2003.

Complete coverage is available in FOXNews.com's Iraq Center.