A top US Republican was among five people who were shot and wounded by a gunman during baseball practice in Virginia.
House of Representatives Majority Whip Steve Scalise was hit but expected to survive, witnesses said.
Police in the upmarket Washington DC suburb of Alexandria said they were investigating a "multiple shooting" and a suspect was in custody.
The lawmakers were practising for a game between Democrats and Republicans.
The annual congressional ballgame was scheduled to be played on Thursday at Nationals Park, home of Major League Baseball franchise the Washington Nationals.
An apparent eyewitness on Twitter, 
Local media report there is a major police presence there after the attack.
Alexandria police tweeted: "Suspect is in custody and not a threat."
'Killing field'
President Donald Trump said in a statement: "We are deeply saddened by this tragedy.
"Our thoughts and prayers are with the members of Congress, their staffs, Capitol Police, first responders, and all others affected."
White House press secretary Sean Spicer tweeted that the president was being updated on the attack.
Who is Steve Scalise?
House Majority Whip, the number three Republican in Congress, responsible for vote gathering
Elected to Congress is 2008 having previously served in the Louisiana state legislature
He apologised for speaking to a white supremacist group during a campaign event in 2002
Former leader of the conservative Republican Study Group, which clashed frequently with former Speaker of the House John Boehner
He married his wife Jennifer in 2005 and has two young children. They live in Jefferson, Louisiana
'A lot of ammo'
Arizona Senator Jeff Flake told reporters he was at bat during the shooting and the victims were "sitting ducks".
He said the attack had lasted for at least 10 minutes, adding: "He had a lot of ammo."
Alexandria Police Chief Michael Brown said officers responded and engaged the shooter within three minutes.
The last member of Congress to be targeted by a gunman was Democrat Gabrielle Giffords, who was shot in the head as she met constituents in Tucson, Arizona, in January 2011. She survived, but six others died in that incident.
Ms Giffords tweeted on Wednesday: "My heart is with my former colleagues, their families & staff, and the US Capitol Police- public servants and heroes today and every day."