Two men have been detained over alleged links to an attack on the American consulate in Istanbul, Turkish television has reported.
The men, who are said to be Turks, were arrested an hour after the attack.
On Wednesday, gunmen opened fire on the consulate before the attack turned into a seven-minute gunfight that killed three officers and three attackers.
Police are still searching for a fourth man they say escaped in a car.
The US ambassador to Turkey has called the attack an obvious act of terrorism.
Al-Qaeda blamed
Meanwhile, CNN Turk TV quoted police sources as saying al-Qaeda was behind the attack on the consulate, Reuters news agency reported on Thursday.
The US said it could not confirm or deny reports about suspected al-Qaeda involvement in the attack.
Sean McCormack, a state department spokesman, said on Wednesday: "I can't say either way. I can't refute them. I can't support them at this point. In terms of our view, it's too early to tell in the investigation."
He said Washington was working closely with the Turkish government "to
determine who is responsible for it and, of course, as best we can, hold to account anybody who was involved" in its planning and execution.
Hoda Abdel-Hamid, Al Jazeera's correspondent in Istanbul, said the attackers jumped out of a car as it pulled close to the US consulate and opened fire.
They were killed when police returned the fire.
The driver of the car fled after the gunfight which lasted seven minutes.
Abdel-Hamid said: "There are several [local news] reports that say that three of the attackers were Turkish nationals and ... have been to Afghanistan at some point. One of them [may be from] south east Turkey.
'Policemen martyred'
"However, I have to be very cautious about this as they [reports] have not been confirmed either by the prosecutor or the police at this stage.
"The fourth attacker managed to flee the scene in his car ... From what we understand the car has been located in a neighbourhood about seven to eight kilometres away from where the US consulate is located. However, this also has not been confirmed."
"Three policemen were martyred and three attackers were killed," Muammer Guler, Istanbul's governor, told reporters at the scene.
Television footage of the attack showed four people lying on the ground at the foot of the consulate's wall, with medics were seen trying to help one of them.
No one has claimed responsibility for the attack.
A US embassy spokeswoman in Ankara said there were no reports of casualties among American consulate employees.
"At about 11am, at least one assailant opened fire on the Turkish police guardpost area near the main entrance to the consulate," she said, speaking anonymously due to the sensitivity of the matter.
"We are co-operating with the police and taking appropriate measures," she said.
Speaking to Al Jazeera, Osman Faruk Logoglu, the former Turkish ambassador to the US, said: "I think it is impossible to say [who was responsible]. What we can say is that this is a terrorist act pure and simple.
"In terms of reasons… there are speculations of all sorts, one line of thinking is that this is a message to the US and their policies in the region. The other suggestion is that it is the PKK.
"Of the four attackers, three were killed, one is still at large and the three dead are all Turkish nationals.
"What they have revealed to us so far (and this has yet to be confirmed) is that they somehow trained in Afghanistan and this leaves a lot of people to jump to the conclusion that there is an al-Qaeda connection.
"This will very much strengthen US-Turkey ties. Definitely security will be stepped up. They have always been sensitive to foreign attacks.
"I can assure you that within hours we may have very strong findings about the perpetrators by the Turkish security forces."
An hour after the attack in Istanbul occured, it was reported that three German tourists had been kidnapped in eastern Turkey.
PHOTO CAPTION:
Two suspects detained
Al-Jazeera