CHICAGO (AP) — President Barack Obama vowed that investigators will get to the bottom of a shooting incident Wednesday at Fort Hood, Texas, seeking to reassure the nation whose sense of security once again has been shaken by mass violence.

In a hastily arranged statement, Obama said he and his team were following the situation closely but that details about what happened at the sprawling Army post were still fluid. He said the shooting brought back painful memories of 2009, when 13 were killed at the same post in the deadliest attack on a domestic military installation in history.

"We're heartbroken that something like this might have happened again," Obama said.

Offering thoughts and prayers to the entire Texas community, Obama pledged to do everything possible to ensure Fort Hood had everything it needed to weather a difficult situation and its aftermath. Standing in front of a black curtain, with an American flag nearby, Obama reflected on the sacrifices that troops stationed at Fort Hood have made — including during multiple tours in Iraq and Afghanistan.

"They serve with valor, they serve with distinction and when they're at their home base, they need to feel safe," Obama said. "We don't yet know what happened tonight, but obviously that sense of safety has been broken once again."

Authorities said four people were dead, including the shooter, and 16 were wounded or injured at the Army base.

"Any shooting is trouble," Obama said.

During his flight back to Washington late Wednesday, the president convened a call with members of his national security team from the White House, the Defense Department and the FBI to discuss the shooting, presidential spokesman Josh Earnest said.

The White House later said Obama directed his team to utilize every resource available to fully investigate the shooting.

In Chicago, the president spoke without notes or prepared remarks in the same room of a steakhouse where he had just met with about 25 donors at a previously scheduled fundraiser for the Democratic National Committee. White House officials quickly pushed tables to the side of the room to make room for Obama to speak to the nation.

Defense Secretary Chuck Hagel, speaking to reporters while in Honolulu to host a conference of Southeast Asian defense leaders, called the shootings a "terrible tragedy." Asked about security improvements in the wake of other shootings at U.S. military bases, Hagel said, "Obviously when we have these kinds of tragedies on our bases, something's not working."

Obama traveled earlier Wednesday to Michigan for an economic event before heading to Chicago.

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 (Photo: AP Photo/Carolyn Kaster)