Several Republican senators and representatives have described terrifying scenes of colleagues coming under heavy fire on the baseball field in Alexandria, Virginia, shortly after 7am, before the gunman was eventually shot by local police officers in a dramatic firefight.

Steve Scalise, a Louisiana congressman and the House Majority Whip, was among those shot during the attack. He was struck in the hip by a bullet which caused internal injuries and bleeding. He dragged himself into the outfield after being shot and is now in a critical condition in a Washington hospital.

Two police officers, a congressional aide and a former aide were also injured in the attack, which has raised fresh questions about gun violence and the vitriolic tone of American political debate.

The gunman, James T. Hodgkinson, a 66-year-old American from the state of Illinois, died of his injuries several hours after the attack. The FBI has said it is too early to say whether the ambush was an act of terrorism or an attempted mass political assassination.

Hodgkinson was a supporter of former Democratic presidential candidate Bernie Sanders and a strident critic of President Donald Trump, according to his social media account, and had been photographed by a local newspaper in 2012 at a protest.

Hodgkinson had reportedly previously been charged with domestic battery in 2006, though the charge was later dismissed. ABC News reported police were also called to his home in March after he discharged a firearm more than 50 times. He was found to have a valid firearm identification card and no charges were laid.

Senator and former Republican presidential candidate Rand Paul was standing in the batting cage at the early morning practice session with at least 20 of his colleagues when the shooting began.

After the first shot rang out, he said, "there was a rapid succession of shots, five or ten shots," he said.

"In the field I see Representative Scalise is shot but moving, and he's trying to drag himself through the dirt out into the outfield.

"There are two staffers... they were laying down. Then I'm seeing the shots hitting the dirt around them. They were kind of making a difficult decision - do we we lay here and stay low and hope he doesn't hit us... or does the shooter just advance and come closer and shoot you?"

Another senator described seeing a staffer attempting to flee the field after being shot.

"One staffer was shot out in the field, and he ran, with a wound, to the dugout," Arizona senator Jeff Flake told CBS News.

"A lot of us dove into the dugout and tried to get as many [people] as we could. But at that point there was firing from behind us from the security detail, from the Capitol Police, and I started yelling back 'are you friendly?' Making sure that was our guy," he said.

"We didn't know if there were other shooters who had us surrounded... we didn't know whether to run or not."

Representative Roger Wiliams, from Texas, hurt his ankle as he scrambled to take cover in the dugout, while a young aide from his office, Zach Barth, was shot in the leg.

Mr Williams, who entered an afternoon press conference on crutches, said he and his colleagues "were sitting ducks out there" and became emotional as he described how the police saved the lives of everyone on the field.

"The thin blue line held today," he said.

Mr Paul also praised the actions of the Capitol Police in bringing the gunman down.

"It would have been a massacre without them," he said.

Alabama congressman Mo Brooks told CNN he and others used a belt to make a tourniquet for the young aide shot in the leg as they hid in the dugout.

"There must have been 50 to 100 shots fired," he said.

South Carolina congressman Jeff Duncan later said he believed he had an interaction with the assailant in the parking lot prior to the shooting taking place. Mr Duncan told reporters he was approached by a man resembling the gunman who asked him whether the players were Democrats or Republicans.

According to his social media accounts, Hodgkinson was a supporter of former Democratic presidential candidate and independent senator Bernie Sanders, and had volunteered on his 2016 campaign.

Mr Sanders made a statement in the senate saying he was "sickened by this despicable act".

"Violence of any kind is unacceptable in our society and I condemn this action in the strongest possible terms," he said.

"Real change can only come about through nonviolent action, and anything else runs against our most deeply held American values."

Hodgkinson had left Illinois for Virginia some time around March, according to Tim Slater, the FBI special agent in charge of the investigation, and had been living nearby in his vehicle. He was in possession of a rifle and handgun.

Mr Slater told the media law enforcement was still trying to determine whether this was a targeted assassination attempt on the Republican congressmen.

"It's too early in the investigation to say one way or the other," he said.

President Trump addressed the nation around five hours after the attack took place, praising the "heroic actions" of the two police officers who took down the assailant even after being wounded themselves.

"Melania and I are grateful for their heroism and praying for the swift recovery of all victims," he said.

He called for unity in the wake of the "brutal, brutal assault".

"We may have our differences, but we do well at times like these to remember that everyone who serves in our nation's capital is here because, above all, they love our country," he said.

"We could all agree that we are blessed to be Americans, that our children deserve to grow up in a nation of safety and peace, and that we are strongest when we are unified, and when we work together for the common good."

Mr Scalise was struck in the hip and was taken to the MedStar Washington Hospital Centre in "good spirits" before he went into surgery. The hospital released a statement that evening saying the bullet has fractured bones and injured internal organs, and that the congressman was in a critical condition and would need further surgery.

The politicians and their staffers were practicing ahead of the Congressional Baseball Game, an annual charity event where representatives from the two major parties play one another. The game is still due to go ahead on Thursday.

The shooting was certain to raise again the contentious issue of gun violence in the United States.

At a news conference near the scene of the shooting, Virginia's Democratic governor, Terry McAuliffe, urged gun control measures. "This is not what today is about, but there are too many guns on the street," Mr McAuliffe said, citing a statistic that 93 Americans are killed with guns daily.

Mr Scalise has been a strong opponent of gun control measures and has earned an "A+" rating from the National Rifle Association - the influential lobby for expanding gun ownership rights. He has co-sponsored legislation to weaken gun control laws in the District of Columbia.

Wednesday's incident was the first shooting of a member of the US Congress since January 2011, when Democratic Representative Gabby Giffords was seriously wounded in an assassination attempt at a gathering of her constituents in Tucson, Arizona.

She survived, but six people were killed. Ms Giffords resigned from Congress and became an activist for gun restrictions.

The shooting occurred on the same day that a gunman at a UPS facility in San Francisco killed three people before turning the gun on himself.

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with Reuters