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ORLANDO, FL –A day after a gunman shot dead 49 people at a gay nightclub in Orlando, police investigated the attacker’s ties to ISIS and Americans grieved over the deadliest mass shooting in U.S. history.

Omar Mateen, 29, of Fort Pierce, Florida, carried an assault rifle and a pistol into the packed Pulse club about 2 a.m. Sunday and started shooting.

In addition to the people killed, he wounded at least 53 others, police said.

During the attack, Mateen called 911 to pledge allegiance to the ISIS terror group and mentioned the Boston Marathon bombers, according to a U.S. official.

After a standoff of about three hours, while people trapped inside the club desperately called and messaged friends and relatives, police crashed into the building with an armored vehicle and stun grenades. They killed Mateen after the rampage — the deadliest terror attack in the United States since 9/11.

“It appears he was organized and well-prepared,” Orlando police Chief John Mina said. Authorities said they haven’t found any accomplices.

Authorities have identified 48 victims of the shooting at an Orlando nightclub, Mayor Buddy Dyer told reporters Monday. They have notified the next of kin of 24 of them, Dyer said.

People other than gunman Omar Mateen are being investigated in relation to the mass shooting at the Pulse nightclub in Orlando, U.S. Attorney Lee Bentley told reporters Monday.

“We don’t know if anyone else will be charged in connection with this crime,” he said, adding that there was “no reason to believe that anyone connected to this crime” was placing the public in imminent danger.

“If anyone else was involved in this crime, they will be prosecuted,” he said.