A lone gunman opened fire on Republican members of the congressional baseball team at a practice field in a Washington suburb Wednesday, using a rifle to shower the field with bullets that struck five people, including Steve Scalise, the majority whip of the House of Representatives.

The New York Times reports that two members of Mr. Scalise’s protective police detail were wounded as they exchanged gunfire with the gunman in what lawmakers described as a chaotic, terror-filled ten minutes that turned the baseball practice into an early-morning nightmare. The police said two of the five people shot were critically wounded.

Standing at second base, Mr. Scalise was struck, in the hip, according to witnesses, and collapsed as the shots rang out, one after another, from behind a chain-link fence near the third-base dugout. Witnesses said Mr. Scalise, of Louisiana, “army crawled” his way toward taller grass as the shooting continued.

Two law enforcement officials identified the gunman as James T. Hodgkinson, 66, from Belleville, Ill., a suburb of St. Louis. The Washington Post first identified Mr. Hodgkinson as the suspect in the shooting.

The police said the gunman was wounded and taken into custody after the gunfight with the Capitol Police security detail and local police officers, who arrived minutes after they received desperate calls for help from those at the field. The F.B.I. said the bureau would take the lead in the investigation, treating it as an assault on a federal officer.

Witnesses described a man with white hair and a beard wielding a long gun standing behind the dugout.

“He was hunting us at that point,” said Representative Mike Bishop, Republican of Michigan, who was standing at home plate when the shooting began at 7:09 a.m. Mr. Bishop said the gunman seemed to be “double-tapping” the trigger of his weapon. “There was so much gunfire, you couldn’t get up and run,” he said. “Pop, pop, pop, pop — it’s a sound I’ll never forget.”

The authorities said they could not comment on the motive for the shooting. Tim Slater, a special agent in charge in the F.B.I’s Washington field office, said investigators are “exploring all angles.” Asked whether the shooting was an assassination attempt, he said it was “too early in the investigation to say one way or another.”

Aides to Mr. Scalise said Wednesday morning that he underwent surgery at MedStar Washington Hospital Center and was in stable condition. The police said a total of five people were transported to hospitals.

A friend of Zachary Barth, a staff member for Representative Roger Williams, Republican of Texas, posted a message from Mr. Barth on Facebook saying: “I got shot this morning at the baseball fields. But I am in the hospital and ok. Thank you for the thoughts and prayers.”

Representative Mo Brooks, Republican of Alabama, told CNN that the gunman, who had a rifle, had said nothing as he opened fire. At least 50 shots were fired, congressional sources said.

Mr. Brooks said he went to the aid of one of the victims and used his belt as a tourniquet to help stop the bleeding from a gunshot to his leg. He said it was about two to five minutes before other officers arrived.

“My adrenaline is raging,” he told CNN. “It’s not easy to take when you see people around you being shot and you don’t have a weapon yourself.”

Mr. Scalise represents the First Congressional District of Louisiana, which extends from the New Orleans suburbs.

Mr. Brooks said Mr. Scalise was shot in the hip and was at second base and dragging himself into the outfield to get away from the gunman.

“There is not a whole lot you can do,” he said. “It was emotionally distressing to know the position he was in.”