After killing the policeman, the gunman “brutally and violently attacked innocent people who came here to enjoy themselves,” said Istanbul’s mayor, Vasip Sahin. Some witnesses, speaking to local media, said they had seen two or more assailants. Security forces later stormed the nightclub. It was the fourth major attack in Turkey in less than a month, including the Dec. 19 assassination of Russia’s ambassador to Turkey. There was no immediate claim of responsibility for the shooting. Turkey has recently taken a central role in trying to halt the hostilities in Syria, in coordination with Russia, which is allied with the government of Syrian President Bashar al-Assad. The attacks have come despite a widening security dragnet by authorities. President Obama was briefed on the assault and has “directed his team to offer appropriate assistance to the Turkish authorities,” White House deputy press secretary Eric Schultz said in a statement.