A female suicide bomber has blown herself up in the Turkish city of Istanbul, killing one police officer and injuring another, officials say.
She targeted a police station in the tourist hub of Sultanahmet, near the Blue Mosque and Hagia Sophia museum.
The woman spoke English with "a thick accent", but her nationality and identity remained unknown, Istanbul governor Vasip Sahin told Turkish TV.
No group has yet said it was behind the attack, the second on police in a week.
Police arrested a man who threw grenades and fired a weapon at officers near the prime minister's office last Thursday, but no-one was injured in that attack.
The bomber in the latest incident died in the attack.
Police sealed off an area in the historic district, where the Blue Mosque and the Hagia Sophia museum are located.
Mr Sahin said the woman, dressed in a niqab, entered the police station and told officers she had lost her wallet before detonating the bomb.
The attack briefly shut down public transport around the historic Sultanahmet square, a popular tourist destination visited by thousands of people every day.
Turkish Prime Minister Ahmet Davutoglu told reporters he had ordered "the most comprehensive investigation" into whether there was a link to any group.
The banned Marxist DHKP-C group said it was behind the earlier attack in the city on 1 January, which took place outside Dolmabahce Palace.
Considered a terrorist organisation by Turkey, the EU and US, DHKP-C has launched several attacks since the 1980s, including on police, security officials and, most recently, the US embassy in Ankara in 2013.
But the group is by no means the only threat Turkey currently faces, says BBC Istanbul correspondent Mark Lowen, referring to recent skirmishes between Kurdish groups and police in the south-east, as well as an overspill of violence from the bloodshed of Syria's civil war.
Turkey is on high alert - and the latest incident is a stark reminder of the many security challenges that this country must now confront, our correspondent adds.