A masked gunman opened fire at a Nashville church, walking silently down the aisle as he shot unsuspecting congregants. At least one person was killed and seven others wounded.

An usher confronted the shooter, who apparently shot himself in the struggle on Sunday before he was arrested, police said.

The FBI said it has opened a civil rights investigation into the shooting at the Burnette Chapel Church of Christ.

No motive was immediately determined. Church members told investigators the suspect had attended services a year or two ago, said Don Aaron, a spokesman for the Metro Nashville Police Department.

Nashville police did not immediately comment on several bizarre posts on the suspect's Facebook page in the hours before the shooting.

The gunman pulled into the church's parking lot as services were ending. He fatally shot a woman who was walking to her vehicle, then entered the rear of the church with two pistols and kept firing, hitting six people, Aaron said. Police said they later recovered another pistol and a rifle from the suspect's car.

Authorities identified the attacker as Emanuel Kidega Samson, 25, of Murfreesboro, who came to the United States from Sudan in 1996 and was a legal US resident.

It was unclear whether the self-inflicted wound to the chest was intentional, Aaron said.

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The gunman was discharged hours later from Vanderbilt University Hospital but remained in police custody. The Metropolitan Nashville police tweeted Sunday night that Samson will be charged with one count of murder and that multiple "additional charges will be placed later."

Witness Minerva Rosa said the usher was "a hero."

"He's amazing," said Rosa, a member of the church for eight years. "Without him, I think it could be worse."

The suspect said nothing as he fired. While the gunman made his way down the aisle, Rosa said, the pastor started shouting, "'Run! Run! Gunshots!'"

Aaron called the usher, 22-year-old Robert Engle, "an extraordinarily brave individual".