The number of people held for suspected links to the deadly New Year’s Eve attack on an Istanbul nightclub increased to 35 on Thursday, according to a police source.

A comprehensive investigation into the terror attack remains underway, the source, who spoke on condition of anonymity due to restrictions on talking to the media, said.

At least 39 people, including a police officer, were killed when an attacker opened fire on partygoers at the Reina nightclub in the Ortakoy district in the early hours of New Year’s Day; 69 other people were also hurt, with a number of foreign nationals among the casualties.