A lethal attack at the entrance of the United States Consulate in Istanbul Wednesday left three Turkish policemen and the three assailants dead.
The US ambassador to Turkey, Ross Wilson, called the strike a "terror attack" and a police official in Istanbul told the Associated Press that authorities suspected that Al Qaeda was behind the strike.
But without a claim of responsibility as of Wednesday morning, the list of potential suspects is long, including an array of groups that have conducted violent acts in the past against both Western and Turkish targets.
Turkey is a favorite of Westerners who flock to see its Ottoman and Byzantine historical sites and immaculate Mediterranean beaches. And the sprawling city of Istanbul, long seen as a symbolic bridge between East and West, has been used for intrigue, violent attacks, and killings by a host of groups: ultranationalists, Kurdish separatists, and Islamist militants.
"I curse strongly these kinds of terror attacks," said President Abdullah Gul on Wednesday, according to state-run Anatolian news agency. "Turkey will struggle to the end with those who organize these [attacks] and the mentality behind them."
The most serious attacks were in November 2003, when 62 people were killed by Islamist militants who targeted two synagogues, a bank, and the British consulate. Since then, security has been tight at most major consulates and embassies.
The US consulate itself is on the outskirts of the city, high on a hill and protected by a large wall. Anyone going in must first enter at a vestibule, walk along a path, take an elevator, and then walk along another hall to come close to any offices. It is one of the most secure locations in the city.
Four people were killed and 15 wounded in an explosion in Istanbul in June 2004, before a visit by President Bush.
Wednesday's attack on the US Consulate comes as Turkey is reeling from revelations that a gang of ultranationalists, called "Ergenekon," had been planning a campaign to sow chaos in Turkish society, to unseat the Islam-rooted government by prompting a military coup.
Those plans reportedly included provoking street clashes with Turkish police and assassinations, all due to begin on July 7.
Istanbul Gov. Muammer Guler said Wednesday's attack was carried out by Turkish nationals, denying reports that the gunmen had Syrian passports.
The US ambassador refused to speculate on the identity of the attackers but stressed that the US and Turkey would keep working together against terrorism.
"It is, of course, inappropriate now to speculate on who may have done this or why. It is an obvious act of terrorism," Mr. Wilson said. "Our countries will stand together and confront this, as we have in the past."
CNN-Turk television reported that some of the attackers had traveled to Afghanistan previously, but police would not confirm the report. In the past, some of the Islamic militants who have operated in Turkey were trained in Al Qaeda camps in Afghanistan.
•Material from the Associated Press and Reuters was used in this article.