ISTANBUL, Turkey -- In poor, seaside neighborhoods of Istanbul, a city straddling Asia and Europe, men in blue jeans mix easily with others wearing headcoverings and long Islamic-style cloaks.
Authorities believe al-Qaida activists or their sympathizers may have established themselves in these conservative areas and run a weeklong terror campaign that has killed 57 people and wounded hundreds more.
Suicide bombers detonated explosives-laden pickup trucks in front of two synagogues, a bank and the British consulate.
Turkish Prime Minister Recep Tayyip Erdogan said Saturday that "citizens with links abroad" carried out the attacks and security forces were pursuing any accomplices.
"I am sure they will be caught," Erdogan said.
A government intelligence report on extremist groups said some Turkish militants who fought in Chechnya, Bosnia and Afghanistan and trained in al-Qaida camps were currently in Turkey, according to published reports.
A likely place to start the search would be the teeming neighborhoods of Istanbul that have become fundamentalist strongholds, a Turkish intelligence official said on condition of anonymity.
Experts say al-Qaida activists might have been attracted to Istanbul because the city is a giant metropolis where religious newcomers would blend in easily with an overwhelmingly Muslim population of more than 12 million.
The city's population has exploded in recent decades as millions of people from the countryside moved into poor, traditional shantytowns ringing the city looking for jobs.
"These people (in religious neighborhoods) might not be interested in supporting terrorist activities but a potential terrorist could melt into the crowd there and not be detected," said Ilter Turan, a political scientist at Istanbul Bilgi University.
Migrants from countries like Iraq, Afghanistan and Iran gather in Istanbul hoping to get into Europe. Police detain dozens of them every week.
Although there is widespread condemnation of the suicide bombings in Istanbul and throughout the country, there is also strong anger toward the United States and Israel in these neighborhoods.
Authorities are concerned that militants could hide in such communities, where they would mix into the society and could find some support for their anti-American or anti-Israeli ideology.
"The fact that this is a big sprawl makes it easy for al-Qaida to hide," said Soner Cagaptay, an analyst with the Washington Institute of Near East Policy.
Balat, a poor, religious neighborhood overlooking the Golden Horn waterway, and Sultanbeyli outside of the city center are two areas where conservative sentiment runs high. In Balat, residents condemned the bombings and said the attackers could not consider themselves proper Muslims.
"There is no possibility that Muslims could be doing this," said resident Mehmet Kayacan, who said Israel most likely was behind the attacks.
Kayacan, a man with a long, graying beard, wore a long gray coat called a cuppe that is popular with fundamentalists.
The attackers "are people who are brainwashed," added Hasan Akbas, a retired construction worker.
Experts say radical groups could have carried out the Istanbul attacks as a message to Turkey -- a staunchly secular, democratic state that sees its future in the European Union, the polar opposite of al-Qaida's world view.
The head of Turkey's national security agency briefed Turkey's top leaders on an intelligence report on extremist groups Friday, the Radikal newspaper reported.
The report said that although Turkey had cracked down on extremist groups in the past, the organizations were regrouping, the Milliyet newspaper reported. The new groups were smaller and more localized but also likely to be more extreme than their predecessors, Milliyet reported.
The report said about 1,000 Turkish militants fought in Chechnya, Bosnia and Afghanistan, and some who trained in al-Qaida camps and fought alongside Afghanistan's ousted Taliban regime currently were in Turkey, Milliyet reported.
An Arab counterterrorism official said there was little evidence of organizational links between Turkish militants and Arab extremist groups.
"It's more likely that some Turks who have trained with Arab Jihadis in bin Laden camps ... have maintained contacts since," the official told The Associated Press.
Kayacan said he heard of Turks going to fight in Chechnya, but doubted that any came back and joined radical groups.
"There are Muslims who probably went to Chechnya but I don't believe that a Muslim who goes to Chechnya to help Muslims would come back and do something like this," he said.