Mayor Bill de Blasio says New York City is deploying more than 500 anti-terrorism officers around the city as a precaution after the deadliest mass shooting in modern U.S. history.

Law enforcement officials were in Penn Station's Amtrak area on Monday with a machine that's used for swabbing bags to check for bomb residue.

Transportation Security Administration officials, police and National Guardsmen dressed in fatigues patrolled the station — a fairly familiar sight since 9/11.

Early Sunday in Orlando, Florida, a gunman opened fire at a nightclub, killing 49 people before police fatally shot him.

De Blasio and Democratic Gov. Andrew Cuomo will attend a vigil Monday at the Stonewall Inn in Manhattan. Hundreds of people gathered there Sunday, with some chanting, "No hate!" and "More love!"