At least 26 people have been killed after a gunman opened fire with an assault rifle at a smalltown Texas church, with the youngest victim just five years old.
The suspect, wearing black tactical gear and a ballistic vest, targeted the First Baptist Church in Sutherland Springs, 30 miles southeast of San Antonio.
Devin Kelley - who served in the Air Force before being thrown out for assaulting his family - attacked while worshippers gathered for Sunday service.
The mass shooting in the town of just 400 or so people is the worst in Texas history.
Donald Trump has described the shooting as an "act of evil".
"Our hearts are broken but in dark times, and these are dark times, such as these, Americans do what they do best," said the President.
The victims range from age five to 72 and include Annabelle Pomeroy, the 14-year-old daughter of the church's pastor.
Crystal Holcombe was killed alongside three of her children and her in-laws, her cousin Nick Uhlig told the Houston Chronicle and Associated Press.
Twenty others were injured, including a six-year-old boy named Rylan who was in surgery after being shot four times, his uncle told CBS.
Wilson County Sheriff Joe Tackitt said there was probably "no way" for people to escape once Kelley opened fire.
"He just walked down the centre aisle, turned around and my understanding was shooting on his way back out," said Sheriff Tackitt.
"It's unbelievable to see children, men and women, laying there. Defenceless people."
As Kelley finished the massacre and tried to escape in his car, a neighbour opened fire and gave chase with another man.
"His vehicle was parked, door open, engine running... him and the neighbour across the street (were) exchanging fire," said Johnnie Langendorff.
"We hit about 95(mph) going down 539 trying to catch this guy until he eventually lost control on his own and went off in a ditch.
"The gentleman with me got out, rested his rifle on my hood and aimed at him, telling him to 'get out, get out'.
"There was no movement, the guy didn't put up a fight or anything."
Kelley, 26, was found dead with several weapons in his vehicle but it is not yet clear if he killed himself or was hit by gunfire.
The shooting came about a month after gunman Stephen Paddock killed 58 people in Las Vegas - the worst shooting in modern US history.
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Mr Trump said the Texas mass murder was not a "guns situation" but a "mental health problem at the highest level".
The President added: "Fortunately somebody else had a gun that was shooting in the opposite direction otherwise it (wouldn't) have been as bad as it was. It would have been much worse."
"We are dealing with the largest mass shooting in our state's history," said Texas Governor Greg Abbott.
He added: "There's so many families who have lost family members. Fathers, mothers, sons, and daughters."
Russia's President Vladimir Putin has sent his condolences to Mr Trump over the shooting, according to the Kremlin.
Authorities have yet to give a motive for the shooting.
An official said Kelley lived in a suburb of San Antonio and did not appear to be linked to any terror groups.
Domestic terrorism is defined by the FBI as an attack by a person or group that "espouse extremist ideologies of a political, religious, social, racial, or environmental nature".
"We don't think he had any connection to this church," Sheriff Tackitt told CNN. "We have no motive."
The US Air Force said he had served in the Logistics Readiness until being discharged in 2014 for bad conduct after being court martialed on charges of assaulting his wife and their child.
A LinkedIn profile that appears to belong to the killer states that he taught bible studies to children at a Baptist church in Kingsville, Texas, for six months in 2013.
Investigators are examining posts he may have made on social media in the days before the shooting, including one that appeared to show an AR-15 semi-automatic weapon.
Kelley's Facebook page has been deleted, but cached photos include one of what appeared to be an assault rifle with the caption: "she's a bad b****".