Had two Capitol Police officers not worked a baseball diamond at 7 a.m. Wednesday, several members of Congress might be dead.
But their quick action and heroics prevented a shooting rampage from becoming what "very well could have been a large-scale massacre," Rep. Mo Brooks said.
The attack proved how vital -- and encompassing -- the Capitol Police can be. Here's what we know about the often obscure police force:
Who are the US Capitol Police, and what do they do?
More than 2,100 officers and civilians work for the Capitol Police. They're tasked with protecting the members, employees, visitors and facilities of Congress, according to their website.
The USCP also collaborates with other federal agencies and local law enforcement "to increase the collection and sharing of intelligence information," according to the Capitol Police strategic plan.
How big is their jurisdiction?
The Capitol Police jurisdiction is in the District of Columbia. But if members of Congress need additional security while in their home districts (or elsewhere), Capitol Police can coordinate with local police departments to provide that security.
Who were the Capitol Police officers in Wednesday's attack?
Special agents Crystal Griner and David Bailey are being hailed as heroes for taking down the gunman who shot House Majority Whip Steve Scalise of Louisiana and several others at a baseball practice.
Griner and Bailey are both members of Scalise's security detail.
Brooks, who was on the field when gunfire rang out, said he was amazed by the bravery of Capitol Police.
"The Capitol security guy, who had already been shot, who helped take down the shooter, came limping toward us in the outfield, totally ignoring his own wounds, to check on the person he was primarily responsible for, Steve Scalise," said Brooks, an Alabama Republican.
"We insisted that he (the officer) go back and get attended to because he was bleeding, but he was doing his job. Those two security details, those two Capitol police officers, they showed incredible bravery -- pistols against a rifle -- from a 90- to 120-feet distance. As you know, that kind of distance heavily favors the rifle."
Who gets personal security detail?
Definitely not all 535 members of Congress. But a handful of congressional leaders do have an officer nearby.
"Only constitutional office holders are given (security) details," said Rep. Joseph Crowley of New York, the Democratic Caucus chairman.
In the House, that includes the speaker of the House, the majority and minority leaders, and both the whips.
"I'm not issued a detail," Crowley said. "No other members of leadership have detail or protection."
In the Senate, the leaders given personal detail are Majority Leader Mitch McConnell, Minority Leader Chuck Schumer, Majority Whip John Cornyn, Minority Whip Dick Durbin and President Pro Tem Orrin Hatch.
But any member of Congress can get extra security, if needed.
"If a threat is made against a specific member, the threat would be reported to police," Crowley said. "Then it would go through a risk assessment, and if it was determined that (the member) needed more security, then they would do it."
What events are Capitol Police required to work?
Capitol Police are required to secure all events on Capitol grounds.
While some members of Congress are granted off-site security detail, the extent to which Capitol Police are present at events outside Capitol grounds is not clear.
Several Congress members declined to provide details on how broad the security detail is, citing security concerns.
What happened to the officers wounded in Wednesday's attack?
Capitol Police Chief Matthew R. Verderosa commended Griner, Bailey and a third officer, Henry Cabrera, "for their heroic and appropriate response in protecting the Members and others."
"I'm grateful that Special Agent Griner is in good condition in the hospital having been shot in the ankle, and Special Agent Bailey was treated and released having sustained a minor injury during the incident," Verderosa said in a statement.
Rep. Roger Williams of Texas was at the GOP baseball practice Wednesday when the shooting started. He praised the two officers who fired back at the gunman.
"There could have easily been 25 deaths today," Williams said. Then his voice began shaking.
"My family and I will be forever grateful."