James T. Hodgkinson — the 66-year-old gunman who attacked Republicans preparing for a baseball game Wednesday morning — posted anti-Trump rants online, defended the political philosophy of Sen. Bernard Sanders and was an outspoken critic of GOP policies.

In one recent Facebook post, Hodgkinson called Mr. Trump a traitor who “destroyed our Democracy.” He called his Republican congressman 10 times over the past year to voice concerns.

Hodgkinson, who had volunteered for Mr. Sanders’ presidential campaign, also wrote against Democratic presidential nominee Hillary Clinton, who he said “stole” the primary from Mr. Sanders.

But the home inspector from Illinois spewed nearly all of his vitriol at Republicans, and when approaching the baseball field Wednesday morning made sure first to ask whether the players were Republicans or Democrats.

Rep. Mike Bost, who represents the Belleville area in Congress, said Hodgkinson’s contacts with his office didn’t seem out of the ordinary.

“While he continually expressed his opposition to the Republican agenda in Congress, the correspondence never appeared threatening or raised concerns that anger would turn to physical action,” the Illinois Republican said in a statement.

Mr. Sanders said he was “sickened by this despicable act.”

Authorities were working to cobble together a full story of what motivated Hodgkinson to shoot and injure a congressman and three others on the Alexandria baseball field. The attack sent members of Congress, staffers and even children scrambling to take cover.

They asked for help understanding why the gunman, who was killed in a firefight with police, left his home in rural Illinois in March and drove to the Washington area, where he was unemployed and living out of his car.

“While the subject is deceased, we continue to actively investigate the shooter’s motives, acquaintances and whereabouts that led to today’s incident,” said Tim Slater, the special agent in charge of the FBI’s Washington field office.

Lawmakers who attended the practice game said Hodgkinson was the same person who had approached them in the parking lot just prior to the shooting.

Rep. Jeff Duncan, South Carolina Republican, described the interaction, saying the man asked “if the team practicing was a Democrat or Republican team.”

“I told him they were Republicans. He said, ‘OK, thanks,’ turned around,” Mr. Duncan said. “I got in the car and left to find out my Republican colleagues were targeted.”

Details about the gunman emerged as the FBI and local law enforcement officials conducted a search of his home in Belleville, just outside St. Louis.

Hodgkinson did have a history of violence and of behavior that drew police attention. Police reports indicate he was arrested on charges of domestic assault more than a decade ago but more recently was the subject of a complaint about gunfire outside his rural Illinois home.

A St. Clair County Sheriff’s Department report from 2006 indicates that Hodgkinson was arrested and charged with domestic battery and aggravated discharge of a firearm for a domestic assault, during which he was accused of firing a shotgun at a man.

The report says Hodgkinson went to a neighbor’s home to retrieve his foster daughter and struck a man who intervened in the head with the wooden stock of his shotgun. When the man turned to run, the report states, Hodgkinson fired once. It was unclear whether he aimed at the man or in the air. Hodgkinson then entered the home and fought physically with his daughter as he attempted to get her out of the house. He left and went back to his home before deputies arrived on the scene.

St. Clair County court records indicate that the charges were later dismissed.

Sheriff’s reports indicate that since then, deputies have responded several times to Hodgkinson’s home to take complaints of damage to his yard.

More recently, deputies had to intervene after they received a complaint of shots fired on his property by a neighbor.

William Schaumleffel, the neighbor who called police, told The Washington Times that he was outside with his grandchildren when he was startled by a series of loud gunshots. Scanning the farmland surrounding his property, he couldn’t immediately determine where the shots were coming from.

“There is a grove of white pine trees, and he stepped out of there and I could see he had a gun he shouldered,” Mr. Schaumleffel said. “He pointed it. It wasn’t at us or the house, and he wound off about six rounds.”

Mr. Schaumleffel said he yelled at his neighbor, who was more than 200 yards away, to knock it off. He got no response and called the sheriff’s department when Hodgkinson kept firing.

“It was too much gun to be firing around the area because of the houses around,” he said.

Deputies said they spoke with Hodgkinson, who had an Illinois firearms owners identification card, and advised him not to discharge the hunting rifle in the area. Possessing a gun and firing it safely in that part of the county is not illegal, so no charges were filed, the sheriff’s office said.

Hodgkinson grew up in Belleville and operated the home construction and inspection business JTH Inspections for decades before closing it last year. In recent years, he wrote a series of letters to the editor, published in the Belleville News-Democrat, in which he railed against Republicans and income inequality and voiced support for tax reform.

Despite Hodgkinson’s eager involvement in politics, his neighbors said he and his family mostly stuck to themselves.

“They just didn’t mix,” Mr. Schaumleffel said.

• S.A. Miller contributed to this report.

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