Public safety officials have outlined a tight security plan for the Boston Marathon three years after two bombs inside backpacks exploded near the finish line.

Authorities said during a news conference today that there will be up to 5,000 law enforcement officers, enhanced checkpoints and dozens of surveillance cameras along the 26.2-mile route on April 18th.

The special agent in charge of the FBI's Boston division says officials aren't aware of specific threat against the marathon. Special agent Hank Shaw says officials are incorporating lessons learned from attacks in Paris, Brussels and San Bernardino, California, into the marathon security plan.

Officials are asking spectators to leave backpacks and drones at home.

Security has been stepped up significantly since the 2013 bombing killed three people and injured more than 260.  (Associated Press)