It's being called a routine training exercise.
But the sight of helicopters landing on the U.S. Capitol grounds Monday -- almost five months to the day after the Jan. 6 assault -- was concerning enough that officials had told nearby residents beforehand, "Please do not be alarmed."
And it comes amid a congressional debate over how to respond to a similar attack.
Working alongside local partners, the U.S. Capitol Police exercise early Monday morning included emergency vehicles coming close to the Capitol and two helicopters flying low across the Washington skyline and then silhouetted against the Capitol dome.
Although Capitol Police regularly train 17 miles away from the Capitol in Cheltenham, Maryland, it's is the first time they've had a full-scale training in years.
The exercise began with K-9 officers patrolling the grounds. Minutes later ambulances filed in, followed by specialized police, emergency, fire and bomb squad units. Several actors were also included.
Several Capitol Police officers told ABC News they'd never seen a similar training on the Capitol grounds. Another long-time Capitol staffer said the last time he'd seen a similar training was shortly after the Sept. 11 attacks.
The first of two helicopters arrived at the Capitol at 7:34 a.m. and was piloted by the Fairfax Police Department.
Shortly after, the second helicopter landed at 7:44 a.m. and was manned by U.S. Park Police. After loading one passenger, the helicopter took off.
The training concluded shortly afterward.
Tensions are still high five months after supporters of former President Donald Trump stormed the U.S. Capitol on Jan. 6. D.C. Police Officer Michael Fanone called the event "the most brutal, savage hand-to-hand combat of my entire life."
Ahead of the training, text messages were also sent to cellphones on Saturday to warn area residents not to be concerned.
Lawmakers are also debating how to address ongoing security matters and whether a quick reaction force is needed. Retired Lt. Gen. Russel Honore oversaw a security review of the Jan. 6 attack and has said that if there was such a force on Jan. 6 the outcome "could have been totally different."
A joint Senate investigation of the Jan. 6 riots is expected to produce a more than 100-page report this week, complete with recommendations on how to prepare for any future attack, including hardening Capitol infrastructure and reorganizing the Capitol Police and its governing board.
It's also been two months since a man "intentionally struck" two Capitol police officers with a vehicle. In that incident, one officer died and the second was injured. The driver was then killed after he rammed his car into a barricade, exited the vehicle and moved towards another Capitol Police officer with a large knife, according to the Metropolitan Police Department.
ABC News' Trish Turner and Lauren King contributed to this report.