At least four people were killed and 14 injured after a gunman opened fire at Fort Hood Army base in Killeen, Texas, The New York Times reports.
AP reported the suspected gunman was among the dead, apparently from a self-inflicted gunshot wound, but a base statement said this was still unconfirmed.
"There has been a shooting at Fort Hood and injuries are reported," said a statement from base public affairs. "Emergency crews are on the scene."
The situation was still considered "active" as military officials told NBC News a possible second possible shooter was being sought.
The post is still under lockdown. People on post are being advised to stay away from windows and doors.
"All personnel on post are asked to shelter in place," wrote the official Twitter account of Fort Hood.
Fort Hood said police had responded to the shooting, which began around 4:25 p.m. local time, according to Ryan Wood, a producer at WFAA Channel 8.
Fort Hood was the same base attacked in 2009 by then-Maj. Nidal Hasan, who opened fire in a predeployment facility, ultimately killing 13 people and wounding 32 others.
President Obama, who was travelling in Chicago and following the situation, told Los Angeles Times "We're heartbroken something like this might have happened again."
At 340 square miles, Fort Hood is the largest U.S. military installation, according to NBC News.
KCEN has a livestream of coverage, and a press conference is soon to be underway at the base.
This story is developing and will be updated as we learn more.