BOSTON (AP) — About 5,000 uniformed and undercover police officers, surveillance drones, bomb- and chemical-sniffing dogs and heavy trucks blocking streets are just some of the security measures being used to protect this year’s Boston Marathon, law enforcement officials said Tuesday.

Authorities are taking no risks when it comes to the 122nd running of the marathon, even though there are no known threats to the April 16 race that goes through eight cities and towns.

“At this point in time, I can tell you neither the FBI, other U.S. government agencies or our state and local law enforcement partners are aware of any specific, active or credible threats directed at this year’s marathon,” said Harold Shaw, special agent in charge of the FBI’s Boston office.

Many of the security measures have been in place since 2013, when two bombs planted near the finish line killed three spectators and wounded more than 260 others.

But police are constantly updating their plans based on world events, and new measures may not always be evident to the public, they said.

Changes to security have been made since the Las Vegas shooting in October and terrorist attacks in Europe in which vehicles were used as weapons, officials said.

Boston police will deploy officers on rooftops near the marathon route, Boston Police Superintendent Bill Ridge said.

Even though there are no known threats to the marathon, police urged the public to remain vigilant and contact law enforcement if they notice anything suspicious, no matter how trivial it may seem.

“We cannot become complacent,” said Transit Police Chief Kenneth Green. “We are asking our riders and our employees to be our second set of eyes. Nothing is too small.”

Boston Athletic Association Chief Executive Thomas Grilk said he’s confident that law enforcement agencies have made the race as safe as possible.