ALEXANDRIA, Va. -- A rifle-wielding attacker opened fire on Republican lawmakers at a congressional baseball practice Wednesday, wounding House GOP Whip Steve Scalise of Louisiana and several others as congressmen and aides dove for cover. The assailant, prepared with "a lot of ammo," fought a gun battle with police before he, too, was shot and later died.

"This is a stable situation," Michael Brown, chief of police for Alexandria, Virginia, told reporters. "At this point, there's no additional threat. We consider this incident to be a closed incident under investigation."

Authorities identified the gunman as James T. Hodgkinson, 66, of Belleville, Illinois.

Late Wednesday, the FBI Special Agent-In-Charge Timothy Slater held a joint press conference with Alexandria police to update its on-going investigation. He said the D.C. medical examiner determined Hodgkinson's cause of death as multiple gunshot wounds to the torso.

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"While the subject is deceased, we continue to actively investigate the shooter's motives, acquaintances and whereabouts that led to today's incident," Slater said. The FBI is asking the public to come forward with any information about the gunman by calling 1-800-CALL-FBI.

Deputy Fred Rogers said the chief commends the special agents who stepped in during the shooting. "Today we saw our our officers' extensive training was put into action."

Dramatic eyewitness video shows the incident unfold.

Gunfire erupts in the background of the video, which was obtained by the New York Post. A player is seen frantically running off of the field as two men ask aloud whether anyone has notified authorities of the shooting.

Sen. Bernie Sanders, I-Vermont, said on the Senate floor that the gunman apparently volunteered on his presidential campaign. Sanders said he was "sickened by this despicable act."

Hodgkinson praised Sanders on social media, CBS News has learned. The FBI is going through his posts, mainly on Facebook, where he was highly critical of President Trump, calling him a traitor. The social media company said his two profiles have been removed from its platform.

A law enforcement source said based on an initial assessment it appears the gunman was increasingly frustrated over the political direction of the country, CBS News senior investigative producer Pat Milton reports. There was no indication that the shooting was inspired by a terrorist group or ideology.

The gunman was believed to have driven to the baseball field in a large white van, Milton reports. The van with Illinois license plates was parked in a nearby parking lot.

Investigators are trying to determine if Hodgkinson was living out of the van. Witnesses have told investigators Hodgkinson was showering at a YMCA adjacent to the baseball field.

Rep. Jeff Duncan, R-South Carolina, told reporters that he had interacted with the gunman Wednesday morning right before the shooting happened.

"I did have an interaction with someone in the parking lot who asked me if the team practicing was a Democrat or Republican team," Duncan told reporters. "I told him they were Republicans. He said, 'K, thanks,' turned around. I got in the car and left, find out that my Republican colleagues were targeted by an active shooter today."

Duncan said that he believes the Republicans were targeted because of their political affiliation.

Sen. Jeff Flake, R-Arizona, who was at the practice, said he got a quick look at the gunman behind a dugout.

"Then when I realized he had a line of sight into the dugout with all the members there then we obviously went back down quickly," Flake said.

The gunman's weapon was similar to an M-4 assault rifle, CBS News has learned. The gunman had a rifle and "a lot of ammo," Flake said.

Scalise dragged himself off the infield leaving a trail of blood as colleagues rushed to his assistance.

Capitol Police officers who were in Scalise's security detail wounded the shooter, who was taken into custody. In all, five people were taken to area hospitals, including the gunman, Alexandria police said. The shooter later died of his injuries, Mr. Trump told the nation from the White House.

"Everyone on that field is a public servant," Mr. Trump said. "Their sacrifice makes democracy possible."

Scalise, 51, the No. 3 House Republican leader first elected to the House in 2008, was shot in the hip, his office said in a statement. House Majority Leader Kevin McCarthy told reporters that Scalise was out of surgery.

MedStar Washington Hospital Center said on Twitter that Scalise was in critical condition. Scalise's office told CBS News that his family was coming from Louisiana to Washington. Before surgery, Scalise spoke to his wife on the phone and was in good spirits, his office said.

Mr. Trump said on Twitter that Scalise "was badly injured but will fully recover."

Rep. Steve Scalise of Louisiana, a true friend and patriot, was badly injured but will fully recover. Our thoughts and prayers are with him. — Donald J. Trump (@realDonaldTrump) June 14, 2017

The other people shot include the gunman, Capitol Police Special Agent Crystal Griner, lobbyist Matt Mika and Zack Barth, a staffer for Rep. Roger Williams, R-Texas.

George Washington University Hospital in Washington, D.C., said one patient it was treating from the shooting had died. The hospital didn't identify the patient, but the announcement came after Mr. Trump said the shooter had died.

The hospital said another patient it was treating was in critical condition. CBS News has identified that patient as Mika, who sustained two gunshot wounds to the chest striking the lung. A former congressional staffer who now lobbies for Tyson Foods, Mika was at the practice as a volunteer coach, CBS News correspondent Kris Van Cleave reports.

U.S. Capitol Police Chief Matthew Verderosa said in a statement that Griner was shot in the ankle and was in good condition. Special Agent David Bailey suffered a minor injury and was treated and released, Verderosa said.

Authorities said in a joint statement that a second congressman sustained minor injuries during the incident. The congressman wasn't identified.

Williams coaches the GOP team, his office said in a statement. He said that Barth "is doing well" and has since been released from the hospital.

Rep. Roger Williams' (R-Texas) injured foot is seen as he speaks about the recent attack against the Republican Congressional baseball team in Alexandria, Virginia, during a news conference on Capitol Hill in Washington, D.C., on Wed., June 14, 2017. Reuters

In a Wednesday evening press briefing, Rep. Williams entered the room on crutches with an ankle injury. He described the shooting as something he will never forget and "there were a lot of heroes out there today." He was on the ballfield when the incident unfolded Wednesday morning.

"I was on the third base side hitting ground balls ... little did I know that the perpetrator was about 20 yards from me," he said.

Williams dived into the first base dugout for cover when shots rang out and when Barth was struck in the leg.

Rep. Roger Williams recounts the shooting and immediate aftermath on Wed., June 14, 2017, during a press conference. CBS News

"Zack had came running in from the outfield ... we landed in each other's arms. He held me, I held him. Jeff Flake took his belt off and made a tourniquet around Zack's leg to stop the bleeding."

"But without our Capitol Police -- who literally took a step forward -- they saved all of us out there. There is no question about it. We had no arms ... we just had bats ... and the shooter," Williams said. "It took the second shot to understand what was happening. We were sitting ducks," he added.

The shocking event left the Capitol horrified and stunned. The House canceled proceedings for the day.

The shooting occurred at a popular park and baseball complex in Alexandria, Virginia, where Republican lawmakers and others were gathered for a morning practice about 7 a.m. They were in good spirits despite the heat and humidity as they prepared for the congressional baseball match that pits Republicans against Democrats. The popular annual face-off, which raises money for charity, was scheduled for Thursday evening at Nationals Park across the Potomac River in Washington.

The team was taking batting practice when gunshots rang out and chaos erupted.

Scalise was fielding balls on second base when he was shot, according to lawmakers present, then dragged himself into the outfield to get away from the gunman.

Rep. Mo Brooks, an Alabama Republican, said his colleague "crawled into the outfield, leaving a trail of blood."

"We started giving him the liquids, I put pressure on his wound in his hip," Brooks said.

Texas Rep. Joe Barton, still in his baseball uniform, told reporters a shooter came out to the practice and opened fire, shooting at Rep. Trent Kelly, R-Miss., who plays third base.

"He shot at Steve Scalise, our second baseman. He hit Steve Scalise," Barton said, "Scalise's security detail and the Capitol Hill police immediately returned fire, and Alexandria Police also immediately came and began to return fire. They shot the shooter. The security detail saved a lot of lives because they attacked the shooter."

Barton said the shooting lasted 5-10 minutes, and there were dozens if not hundreds of shots fired.

"It was scary," Barton said.

Lawmakers took cover in the dugout. Barton said his son, Jack, got under an SUV.

FBI special agent in charge Tim Slater said it was "too early to say" whether it was an act of terrorism, or whether Scalise was targeted.

After the gunfire stopped, Flake said he ran onto the field and also tried to come to Scalise's aide. After medical personnel arrived, he said he retrieved Scalise's phone and made the first call to Scalise's wife to notify her of the shooting. He said he did so to ensure that Mrs. Scalise would not find out about the shooting through the media.

Flake estimated that more than 50 shots were fired. He said the gunman was initially out in the open.

"Marty, our photographer, saw him raise the gun for the first time, the rifle, but he said he thought, 'Why in the world is somebody bird-hunting out here at this time,'" Flake said.

A spokesman for President Obama confirmed to CBS News that Mr. Obama called Flake on Wednesday.

Scalise, a popular and gregarious lawmaker, is known for his love of baseball and handed out commemorative baseball bats to fellow lawmakers when he secured the No. 3 job of House whip several years ago.

Falisa Peoples was just leaving the YMCA next to the ballfield when she saw the shooter open fire.

"He was just very calm. He was just walking and shooting," she said of the man, whom she described as white and wearing a T-shirt and shorts. She said he was using a long gun and exchanging fire with law enforcement officers, one of whom yelled for her to get down.

According to a source, when Democratic members of Congress practicing at a different ballfield miles away heard of the shooting, they immediately stopped and said a prayer together, Milton reports.

In a brief interview in a Senate hallway, Senate Majority Leader Mitch McConnell said, "I think everybody handled it well and things seem to be under control."

Other lawmakers were stunned in the aftermath of the event, which raised questions about the security of members of Congress. While the top lawmakers, including Scalise, have security details, others do not and regularly appear in public without protection. The last time a lawmaker was shot was when Democratic Rep. Gabby Giffords of Arizona was hit in the head and grievously injured while meeting with constituents at a supermarket parking lot in 2011.

Following the Giffords shooting, lawmakers have held fewer open town halls and have been advised to increase security at such events.