Turkish police have arrested five people suspected of direct links to the suicide bombing that killed 10 Germans in Istanbul's main tourist area.
More than a dozen other suspected IS militants were detained on Wednesday in other parts of the country and 59 a day earlier, but officials say they do not appear to be tied to yesterday's attack just steps away from the Blue Mosque in Istanbul's historic Sultanahmet district.
One suspect with a link to the attack was detained in Istanbul late on Tuesday, Turkey's interior minister Efkan Ala said during a news conference with his German counterpart.
Turkish media said police raided a home in an affluent Istanbul neighbourhood, briefly detaining one woman suspected of links to the Islamic State group, although it was not clear if she was the suspect Mr Ala was referring to.
The Hurriyet newspaper said the woman was detained because a mobile phone - which she had reported stolen - had been used to call the bomber.
The paper said she was released after questioning.
Hours later, Prime Minister Ahmet Davutoglu announced the arrest of four other people as part of the investigation.
Turkish authorities have identified Tuesday's bomber as a Syrian man born in 1988 who had recently entered Turkey, and officials say he had links to the so-called "Islamic State" group.
The group, however, has not claimed responsibility for the attack.
Turkish media, including newspapers close to the government, identified the bomber as Nabil Fadli and said he was born in Saudi Arabia.
Mr Ala confirmed reports that the bomber had registered with a refugee agency, providing fingerprints that allowed officials to quickly identify him, but said the suicide bomber was not on any Turkish or international watch lists for IS militants.
The impact of Tuesday's attack, while not as deadly as two others last year, was particularly far-reaching because it struck at Turkey's tourism industry, which has already suffered from a steep decline in Russian visitors since Turkey shot down a Russian warplane near the Syrian border in November.
Turkish police on Wednesday arrested 13 more suspected IS militants, including three Russians, but it was not clear if those arrests were directly linked to the Istanbul bombing.
The state-run Anadolu Agency said 59 people were detained on Tuesday.
The attack, which also wounded 15 other people - including Germans, a Norwegian man and a Peruvian woman - was the latest in a string of attacks by Islamic extremists targeting Westerners.
On Wednesday, Mr Davutoglu, German interior minister Thomas de Maiziere and other Turkish officials visited the site of the blast and placed carnations there.
Germany sent a team of investigators to Istanbul on Wednesday from its Federal Criminal Police Office, which is comparable to the FBI, to support Turkish authorities investigating the attack.
Germany's Foreign Ministry said the number of dead Germans in Tuesday's explosion had risen to 10, but Mr de Maiziere said there was no sign that Germans were specifically targeted.
"According to the investigations so far, there are no indications that the attack was directed specifically against Germans, so there can't be any connection to our contribution to the fight against international terrorism," Mr de Maiziere said.
Germany promised Tornado reconnaissance jets to aid military effort against the Islamic State group in Syria following the November attacks in Paris, and they started flying missions from the Incirlik air base in Turkey last week.
It also sent a tanker aircraft, as well as a frigate to help protect a French aircraft carrier in the eastern Mediterranean.
Germany was already helping supply and train Kurdish forces fighting IS in northern Iraq but has not taken a direct combat role.
Mr Ala urged Turkish citizens and visitors to go about their daily lives, insisting that the country had taken "all necessary security precautions".
He said Turkey had detained as many as 220 IS suspects in the week prior to the attack.
Mr de Maiziere also said: "I see no reason to refrain from traveling to Turkey" or for people already there to break off their vacations.
Top German and Turkish officials were already scheduled to meet in Berlin next week to discuss Europe's migrant crisis, in which Turkey - which borders both Syria and the European Union - is a key player.
Mr de Maiziere said those talks will now also address "the determined fight against terrorism".
"If the terrorists aimed to destroy or endanger the cooperation between partners, then they achieved the opposite," Mr de Maiziere said.
"Germany and Turkey are coming even closer together."