Iraqi police escort suspected militants during a raid operation in the town of Mussayab, 60 km (40 miles) south of Baghdad, September 23, 2009. [Agencies]

TIKRIT, Iraq: Iraqi authorities imposed a vehicle ban and a curfew in a city north of Baghdad on Thursday after more than a dozen suspected al Qaeda militants climbed through a ventilation duct and broke free from jail.

Police officials said security forces were scouring the city of Tikrit, 150 km (95 miles) north of Baghdad, for the fugitive prisoners, some of whom had been sentenced to death for working with Sunni Islamist group al Qaeda.

The escape, in which at least one local official said police may have played a role, raises additional questions about Iraq's security forces and its penitentiary system as US troops prepare to withdraw from the country by the end of 2011.

The men had been convicted of various crimes, including murder, kidnapping and planting roadside bombs.

One of those still missing was Waleed Ayash, an al Qaeda leader accused of killing police and civilians in the town of Dhuluiya in Salahuddin province.

Guards discovered the escape during their rounds in the local jail and found a ventilation duct that had been left open in the main inmates hall.

Mutashar Hussain Allawi, governor of Salahuddin, said an investigation had been opened into the matter and that it appeared there may have been police involvement or negligence.

Iraq's overburdened prison system, known as fertile ground for insurgent recruitment, is facing a new challenge as American forces hand over detainees they have held for years in Iraq.