ANTIOCH, Tennessee -- A gunman opened fire at a church in Antioch, Tennessee, on Sunday, killing one person and wounding six others, according to a spokesman for the Metropolitan Nashville Police Department. An eighth victim was "pistol whipped," police said.
"This is a mass casualty situation," the Nashville Fire Department said in a tweet. "All of the wounded have been transported to area hospitals. The majority are older adults."
Police identified the gunman in a Sunday press conference as 25-year-old Emanuel Kidega Samson.
Authorities are still investigating a potential motive for the shooting, said MNPD chief Steve Anderson, but it could have been carried out for "a variety of reasons," he said.
The shooting occurred at the Burnette Chapel Church of Christ in Antioch, a neighborhood in Davidson County governed by Nashville.
The MNPD said it received the first call of multiple shots fired at 11:15 a.m. CT.
Parishioner confronted, struggled with gunman
The gunman was in the church's parking lot as a prayer service was letting out, said Don Aaron, spokesman for the police department.
The gunman, described by police as an African-American man, shot and killed a woman who was walking to her car before entering the church, Aaron said.
Once inside, the gunman "began indiscriminately shooting," according to Aaron. There were approximately 50 people still inside the sanctuary when the shooting began.
Six people were wounded by gunfire and another was pistol whipped, he added.
A church usher -- later identified by police as 22-year-old Robert Engle -- witnessed the shooting inside the church and "ran up and confronted" the gunman, Aaron said. During a struggle, the gunman mistakenly shot himself, Aaron said.
"It would appear he was not expecting a brave individual like the church usher to initiate the struggle and confrontation," Aaron said.
The suspect is being treated at Vanderbilt University Medical Center under police guard. "It is our belief that the gunman's condition is not life-threatening," Aaron said.
All of the victims have been taken to area hospitals, he added.
Aaron said at a later press conference the victims wounded by gunfire ranged in age from 60 to 84, with the exception of Engle, police said.
Police said the investigation into the shooting is ongoing.
Samson is to be released from Vanderbilt University Medical Center into police custody after receiving medical attention. Aaron said a press conference Sunday afternoon that Samson "will be jailed tonight on a number of very serious charges -- at least one homicide warrant and multiple attempted homicide warrants."
Agents from the FBI's Memphis field office are on the scene providing assistance to local law enforcement, an FBI spokesperson told CNN.
Local police also requested the assistance of the Bureau of Alcohol, Tobacco, Firearms and Explosives to run an "urgent trace" on a firearm, an ATF spokesperson told CNN.
The trace would give authorities an indication of where the gun was purchased, who purchased it and whether it was purchased legally. There are about a dozen ATF agents on the scene, the spokesperson said.
Nashville mayor Megan Barry said in a tweet that the shooting was "a terrible tragedy for our city."
"My heart aches for the family and friends of the deceased as well as for the wounded victims and their loved ones," she said. "Their lives have been forever changed, as has the life of their faith community at Burnette Chapel Church of Christ."
Witness: Gunman didn't say anything
Minerva Rosa, who was inside the church when the shooting started, told reporters Sunday the gunman was silent as he opened fire.
"He don't say nothing," she said. "He just -- he was shooting."
Rosa said the usher tackled the gunman. "Without him I think it could be worse," she said. "He was a hero today."
MNPD chief Anderson echoed the sentiment in a later press conference.
"He's the hero," Anderson said. "He's the person that stopped this madness, so we're very, very, very grateful to him."
Joe Love, a man who lives near the church, did not see the shooting, but told CNN affiliate WKRN he became aware of it when a frantic man and his wife who had both been inside the church came to his door on Sunday.
"The guy shot at them, but they was able to get out," Love said. The couple was not hit.
Love said he raced over to the church, but the gunman had already shot himself. He recognized the woman who was killed in the parking lot, he said.
"Every time that church door open ... that young woman was there," said Love, who is not a parishioner.
"The deceased woman's sister, she run to her car, and I helped her cover her sister up with some towels so she wouldn't be disgraced anymore," Love told WKRN.