The shooter is alleged to have arrived in Istanbul in November. Police have released a video of the suspect and said they have fingerprints that link him to the deaths of 39 revellers at the Reina nightclub.
Where is the gunman from?
It is believed he is from Kyrgyzstan. His wife allegedly told police that they landed in Istanbul from Kyrgyzstan late in November. They moved to Konya, in central Turkey by December and paid three months rent upfront.
The wife, who was not identified, said the suspect tried to find work in Konya.
Turkey's IHA news agency reported that his wife learned of the incident on television. She said she has no information on his whereabouts. She never heard him speak of Daesh.
Any official confirmation?
Kyrgyzstan's foreign ministry said they were looking into the allegations.
"We have ordered the consul in Istanbul to check this report that appeared in the press," ministry spokesman Aiymkan Kulukeyeva said.
A copy of the suspect's passport has been released, but Turkish authorities had not yet confirmed its authenticity.
How did he get to the Reina nightclub?
Police said he took a taxi from Zeytinburnu, a neighbourhood near the Istanbul airport.
He got out of the vehicle in Ortakoy, about four minutes away from the nightclub. Due to the heavy traffic, he had to walk to the nightclub. It is alleged he was carrying a suitcase. Reportedly, the suitcase was used to transport the murder weapon.
What do we know about what happened at the nightclub?
Footage from the nightclub shows the assailant wearing a green shirt and dark trousers. He fired a long-barreled weapon that he aimed at people's upper bodies. It is alleged that he first went upstairs and started shooting at partygoers before heading down to the lower level. Witnesses said that he deliberately aimed at people who took cover by lying on the ground.
During the attack, which is believed to have lasted seven minutes, he changed six magazines and fired more than 180 bullets.
After the shooting spree, video footage showed that he went to the nightclub's kitchen for approximately 13 minutes. It was there that he changed his clothes and removed a coat. Police found 500 Turkish Lira inside the coat pocket. A rifle was found at the nightclub.
Some 600 revellers were in the Reina nightclub on Sunday morning when the gunman struck. Thirty-nine people died and dozens were injured.
How did he flee the scene?
Investigators said he took advantage of the ensuing chaos and slipped out of the club. It is alleged that he got into a cab. The taxi driver told police that he borrowed his phone and made a call. The details of the phone call have not yet been released.
The gunman is said to have exited the taxi in Kurucesme, a short distance from Reina, after telling the driver that he had no money for the fare.
Turkish police have arrested 12 suspects and a massive manhunt is underway for the gunman.
Daesh has claimed responsibility for the attack, saying that it was revenge for Turkey's role in Syria. This is the third major assault carried out by Daesh in Turkey in the past year. In June, they claimed responsibility for the attack on Istanbul's biggest airport, killing 35 and in August, a suicide bomber targeted a wedding in Gaziantep, in the south east, killing 54 people.
Why would Daesh target Turkey?
Turkey launched Operation Euphrates Shield in August after the Gaziantep suicide attack. The operation is aimed at ridding Daesh and other terror groups from Turkey's border with Syria. Ankara is also cracking down on Daesh networks at home. In counter-terrorism operations between December 26 and January 2, Turkish police detained 147 people with suspected links to the group and arrested 25.
Turkey's Deputy Prime Minister Numan Kurtulmus said on Monday that Ankara would continue its operations against terrorists in Syria despite the Istanbul nightclub killings.