Istanbul, Turkey ? Anti-terrorism police detained more suspects Friday and are now holding about a dozen people in a spate of suicide bombings blamed on al-Qaida that claimed 54 lives. An intelligence report presented to authorities said some Islamic radicals who fought in Chechnya, Afghanistan and Bosnia might have returned to Turkey to work with militant groups.

Foreign governments, including the United States and Britain, cautioned that terrorists could strike again and issued travel warnings to their citizens.

The Hurriyet newspaper said police were questioning seven people in connection with Thursday’s blasts at the British Consulate and the Turkish headquarters of the HSBC bank that killed 27 people and two bombers.

At least three groups or individuals citing links to al-Qaida claimed responsibility for the attacks, including a small militant Turkish group, the Islamic Great Eastern Raiders’ Front, or IBDA-C.

Foreign Minister Abdullah Gul confirmed that police made arrests in Thursday’s bombing, but gave no details.

The attacks came just days after bombings at two Istanbul synagogues killed 23 people in addition to the two suicide bombers.