Three years after two bombs exploded near the finish line of the Boston Marathon, public safety officials have outlined a tight security plan for this year's race.

During a news conference Wednesday, authorities said there will be up to 5,000 law enforcement officers, enhanced security checkpoints and dozens of surveillance cameras along the 26.2-mile route on April 18.

Hank Shaw, special agent in charge of the FBI's Boston division, said officials are unaware of any specific threat against the marathon. He said 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.