Tensions are mounting in Istanbul as the search for the gunman who killed 39 people in the Reina nightclub enters its fifth day.
A video posted on social media showed a group assaulting a man they thought was the attacker.
The victim was taken to a police station, then released.
Mehdi Eker, deputy chair of Turkey's ruling AK party, says catching the shooter is only a matter of time.
He's told the BBC he rejects criticism that Turkey's purging of military and intelligence offices following a failed coup in July has led to a weaker search effort.
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"The Turkish intelligence services and the security forces are doing their job quite well, and they're on the run and they will get it. It has nothing to do with the allegation that there is a purge, which we deny completely."
Local media reported 27 people, including women and children, had been detained in the coastal city of Izmir over suspected links with the nightclub attacker.
Those arrests are in addition to at least 12 made earlier in the week.
Authorities say they now know the identity of the suspected I-S attacker, but have not released his name.
Turkish president Recep Tayyip Erdogan has made his first public comments about the attack, saying people linking the attack with the lifestyle choices of nightclub-goers are dividing the country.
Turkey's Religious Affairs Directorate has condemned the attack, but Mr Erdogan appeared to refer to their statement in December that said celebrating the New Year would be out of step with Muslim values.
"Nobody's lifestyle is under systematic threat in Turkey. We will never allow this. We haven't allowed this since we took the helm 14 years ago. Anyone who claims otherwise must prove it with concrete examples."
This week parliament voted to extend by a further three months a state of emergency in Turkey, implemented after the failed military coup in July.
The country is also resisting pressure from the European Union to change its counter-terrorism laws.
Turkish EU minister Omer Celik says Turkey needs its wide-reaching terror laws now more than ever.
"Turkey shares a 1,295 kilometre-long border with Syria and Iraq. Asking a country that faces attacks like that on Reina recently to change its anti-terror laws would put both Turkey's and Europe's security in danger. This is not the right approach."
After the tragic start to 2017, some Turks say they are now afraid to visit crowded spaces like bars and public squares.
One foreigner living in Turkey says locals are getting used to living with the threat of violence.
"I think that our generation has gotten accustomed, in a way, to these kinds of events. They became somehow - unfortunately - the new normal. And I don't think that my life is going to change or be affected by it. I refuse to let my life be changed by it, actually."
