A Turkish court on Sunday ordered a man to be held in custody over an attack on the U.S. consulate in Istanbul in which six people were killed, the state-run Anatolian news agency reported.

The court ordered that the man, identified only as Dursun P., be held on suspicion of belonging to a terrorist group while prosecutors continued questioning nine other people and prepared an indictment.

Two people were released but might still face prosecution, Anatolian said.

The suspects were being questioned about Wednesday's attack in which three police officers and three attackers were killed when gunmen opened fire on security personnel at the entrance to the high-security consulate complex.

A man suspected to have driven the car used by the attackers was detained on Thursday.

Police were investigating possible al Qaeda involvement - newspapers said the gunmen received weapons training in Afghanistan. Some security experts were sceptical about an al Qaeda link, given the small scale and amateurish nature of the attack.

Political tension has been rising in Turkey. The ruling AK Party faces a threat of being banned for alleged anti-secular activities and police are investigating a shadowy far-right group suspected of plotting a coup.

A variety of groups have carried out violent attacks in Turkey in recent years, including left-wing, Kurdish and Islamist militants.

The most serious occurred in November 2003 when 62 people were killed by Islamists who attacked two synagogues, a bank and the British consulate in Istanbul.

The U.S. consulate had been moved to a more secure location in Istanbul after the September 11 al Qaeda attacks in the United States.