NYPD prepares for possible US Open security threats

Nearly one million spectators are expected to attend the US Open, which begins Monday in Queens, New York. Although a new assessment warns that large events "remain attractive targets for attacks by violent extremists and malicious actors," the NYPD said there are currently no known causes for concern.

"There are no specific or credible threats to the US Open. However, we're in a different threat environment than we were last year at this time. And so when we are collectively thinking through all of the measures that we're putting in place around this US Open, we are mindful of what's happening around the world," said NYPD Deputy Commissioner of Counterterrorism Rebecca Weiner at a Monday press conference.

"We recognize that this is an environment where there's a lot of protest activity," Weiner added.

The focal point is Arthur Ashe Stadium, where the US Open will be played, but suspects could use "a broad range of tactics – including elevated position gunfire, vehicle ramming, edged weapons, and improvised explosive/incendiary devices (IED/IID), etc. – at a congested chokepoint areas, such as the Flushing Meadows pedestrian bridge between the Mets-Willets Point train station and the East Gate entrance of the USTA Tennis Center, requiring elevated situational awareness," according to the threat assessment document obtained by ABC News.

Drones will hover over elevated subway lines to monitor for threats, according to officials. Police will also deploy dogs, explosive detection equipment and the bomb squad during the event.

"We cannot overstate how important it is for the people to be engaged, invested and aware. So if you are here at the US Open and you see anything that does not look right or does not feel right, call 911. Tell a police officer right away," Police Commissioner Edward Caban said at the press conference.

Two protesters were arrested at last year's US Open when they began shouting and one glued his feet to the ground during a match between Coco Gauff and Karolina Muchova. Another spectator was ejected from a game a few days earlier after German player Alexander Zverev accused the man of quoting a phrase from Adolf Hitler's Nazi regime.