US President Donald Trump issued a statement to say he is monitoring the shooting.
"The Vice-President and I are aware of the shooting incident in Virginia and are monitoring developments closely. 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."
Katie Fillus of Alexandria had just gotten out of her car to walk her dogs in the park nearby when she said she heard "very, very loud popping sounds", reported The Washington Post.
"And I knew a baseball team was practicing, and everybody started screaming, 'Hit the ground! Hit the ground!' "
She said she lay flat in the field as the gunshots grew louder - "like he was walking across the field toward all of us, the gunman, and I was screaming: 'Can someone help me? I have my dogs and I can't get behind anything.' "
Fillus said a police agent pulled out a gun and tried to shoot back. She was screaming, " 'Drop your weapon!' And he shot her and she fell on the ground.
"She fell on the ground in front of us, and we were all just trying to lay as flat we could. And I belly crawled, dragging through the mud. I got to the car and I ducked under the car and I laid as close as I could under the car to hide from the person. Then the police seemed to come."
Susanne Stratton, a 28-year-old Alexandria resident, was playing with her dog in the dog park next to the baseball field when the shooting began.
"We heard people yelling to get down. We saw people running, some into the dog park, some jumping over the fence," she said. She said the people in the dog park immediately got down on the ground and pushed their dogs down, as well.
She said there was a burst of shots, then a brief pause, then more shots - she estimated about 20. "It must have been a semi-automatic," she said.
Reba Winstead, 43, who lives on street adjacent to ballpark, said she heard about 30 shots fired in bursts and saw two people running down her street in exercise clothes.
"One of the bullets whizzed down our street. That's when I jumped inside, when I heard the whiz," she said. "It's just scary, because you don't hear shots fired in Alexandria very often."
Charles Halloran, who lives in Del Ray about a block from the park, arrived at a YMCA at 7.30am next to the baseball field, to discover the scene unfolding.
"Bullet holes in the glass and people were shaking," Halloran, a former congressional staffer, said in a telephone interview from inside the YMCA. Bullets went through the YMCA's building and across the building into the pool.
Reports of violence are extremely rare in Del Ray, a quiet, upscale neighborhood known for its shops and art and craftsman-style homes.
House Speaker Paul Ryan, a Wisconsin Republican, was at the gym on Capitol Hill when he was informed of the shooting, according to two GOP lawmakers who saw him there. They said he immediately stopped his workout and headed out, guided by his own Capitol Police detail, which is always at his side. The lawmakers spoke on the condition of anonymity to discuss the speaker's movements.
As Ryan left - and Capitol Police briefly told members about the shooting - the gym quickly quieted and members packed up their belongings to also head to their offices, the lawmakers said.
"Nobody knew what the hell was going on," one of the Republicans said. "People just left."
There was one notable departure from the normal level of protection inside the building: Three uniformed officers stood watch outside the first-floor office of House Majority Leader Kevin McCarthy, a California Republican.
