The San Antonio FBI branch said its agents had been deployed while there was no indication of the gunman's motive.
Megan Posey, a spokeswoman for Connally Memorial Medical Center in Floresville, Texas, told The New York Times she did not know exactly how many patients the hospital had received, but more were continuing to arrive.
Photos taken by local media showed several police and emergency vehicles, as well as helicopters, outside the church.
Sutherland Springs is in Wilson County, about 30 miles east of San Antonio.
Sunday's attack comes just a month after the one of the deadliest mass shootings in U.S. history, when 58-year-old Stephen Paddock killed 59 people and at least 527 others.
USA media report children were among the victims.
There have been reports 15 people have been killed.
We will focus on reports from police officials and other authorities, credible news outlets and reporters who are at the scene.
President Donald Trump said he was monitoring the situation while in Japan on a 12-day Asian trip.
A County Commissioner in Texas says he's been told that more than 20 people were killed and more than 20 were wounded in an attack at a church, though he says those figures haven't been confirmed.
Dana Fletcher, who owns a business down the road from the church, told CNN's Fredricka Whitfield that Sutherland Springs is a "very small" but "very tight-knit community".
Texas Gov. Greg Abbott called the shooting an "evil act".
"It was rapid fire", Matula said.
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