[UPDATE 12:45 p.m.] New York City Hall will be lit in rainbow colors Sunday night to represent LGBT pride, and all flags have been ordered to half-staff immediately. In a statement, Mayor de Blasio's Office called the order "a mark of respect for the victims of the Orlando, Florida shooting" and confirmed that the New York State and City flags, the Pride flag, and POW-MIA flags on all City buildings and stationary flagstaffs are to be lowered to half-staff.

The NYPD's counter-terrorism, patrol, and emergency resources have been placed on alert after a mass shooting by a single gunman at an Orlando nightclub left as many as 50 people dead Sunday morning.

The police department has released a statement saying that their "thoughts and prayers are with all of those affected by the senseless shooting in Orlando, Florida." In addition, the NYPD is in contact with local law enforcement in Florida, along with the FBI. Currently, the NYPD's Counterterrorism Response Command, Strategic Response Group, and Emergency Service Unit are on heightened alert.

"New Yorkers should know that the NYPD has more than 500 officers dedicated solely to terror response, hundreds more to the prevention of incidents, and is constantly working with law enforcement and community partners to counter terror," the police said.

Orlando Police Chief John Mina identified Omar Mateen, an American-born citizen from St. Lucie, Florida, as the lone gunman in Sunday's shooting, which took place at Pulse, a gay nightclub in downtown Orlando. Mateen opened fire inside the club at approximately 2 a.m., and was killed in a shootout with law enforcement, the New York Times reports.

"The shooter took hostages and at about 5 a.m. we entered and exchanged gunfire with the suspect and he was dead at the scene," Mina said, adding police used a "controlled explosion" at one point to distract the shooter. "Nine officers were involved and one of our officers was injured. At least 30 people were saved during the rescue." The Washington Post reports that 50 people have been killed, while another 53 people were injured in the attacks. Some expect those totals to rise in the coming hours.

The NYPD's counter-terrorism CRC and SRG were similarly placed on alert immediately following last November's Paris attacks and March's attacks in Brussels.

A memorial vigil for victims of the Orlando massacre will take place outside Stonewall Inn Monday night at 6 p.m.