Turkey arrested one person, the interior minister said following investigations into the suicide bombing which killed 10 Germans in the tourist heart of Istanbul.
Germany's Foreign Ministry confirmed all 10 dead in the blast are German citizens.
Regional officials in Germany say the victims were mostly older, retired people on holiday in Turkey. Turkish authorities are blaming Islamic State for the attack.
The person was arrested on Tuesday, directly following the attack, which also injured 17 people, Interior Minister Efkan Ala said at a joint press conference in Istanbul with his German counterpart Thomas de Maiziere.
Eleven injured remain in hospital, including two Germans in critical condition.
"Based on the current state of the investigation, there is no evidence the attack was directed specifically against Germans," the German minister said, calling this an "attack against humanity."
Turkey - which has faced criticism it has been slow to tackle Islamic State - has arrested more than 3,300 people in operations against the extremist group, including 220 people in the week prior to the attack, Ala said.
Earlier, the Dogan news agency reported that since the attack Turkish police arrested nine alleged members of Islamic State in the coastal Izmir province and southern Antalya province. Among them were three Russian citizens.
Turkey's economy is heavily dependent on its tourism sector and there is some concern the attack could have a negative effect. Germans make up the largest group of visitors to Turkey.
Tourism from Russia, the second largest supplier of visitors, is already in a tailspin following the decline in the Russian economy and worsening relations between Moscow and Ankara after Turkey downed a Russian jet near the Syrian border in November.
Turkish Prime Minister Ahmet Davutoglu, accompanied by his wife, visited the scene of the suicide bombing and laid red carnations at the site.
No group has yet claimed responsibility for the Istanbul attack. Turkish authorities blamed two major suicide bomb attacks last year on Islamic State militants, though the Sunni extremist group did not claim these, unlike most of its international terrorist operations.
Turkish officials said a man who crossed into the country from Syria carried out the bombing Tuesday morning in the Sultanahmet area, a historic part of the city home to the famed Blue Mosque and Hagia Sophia museum.
Ala said the authorities in Istanbul had taken the fingerprints of the attacker a week before the blast. This was apparently done at an immigration office. Turkey hosts approximately 2 million Syrian refugees.
The Saudi-owned al-Hayat newspaper has reported that the alleged suicide bomber, 27-year-old Nabil Fadli, was born in Saudi Arabia but left with his family when he was 8 years old. He held Syrian citizenship, the report added.
Turkey last year stepped up efforts to tackle Islamic State, which holds territory in neighbouring Iraq and Syria, after a period in which the government was criticized for being slow to counter the militia and close its porous Syrian border.
Hurriyet newspaper said the country's intelligence services had warned the security forces twice in the weeks before the attack about the possibility of a suicide attack targeting foreigners.
German Chancellor Angela Merkel, who condemned the "murderous act" in Istanbul, has spoken with Turkish President Recep Tayyip Erdogan by telephone.