The Rev. Jeff Miller lights four candles, one for each of the people killed in the Fort Hood shooting, including the gunman, at the beginning of the worship service at First United Methodist Church in Killeen, Tex. Investigators say that an Army truck driver, Spec. Ivan A. Lopez, had an argument before opening fire at the Texas post on April 2, killing three soldiers, wounding 16 and then taking his own life.

The Rev. Jeff Miller lights four candles, one for each of the people killed in the Fort Hood shooting, including the gunman, at the beginning of the worship service at First United Methodist Church in Killeen, Tex. Investigators say that an Army truck driver, Spec. Ivan A. Lopez, had an argument before opening fire at the Texas post on April 2, killing three soldiers, wounding 16 and then taking his own life. Jay Janner/AP

The 34-year-old soldier who went on a shooting rampage at Fort Hood, Tex., argued with members of his unit over an application for a leave of absence, witnesses said. Spec. Ivan A. Lopez had been undergoing treatment for depression and had seen a psychiatrist last month, but Army officials said they had not considered him a threat.

A shooting at the Fort Hood military installation in Texas has left at least four people dead, including the gunman, and more than a dozen injured, according to authorities.

A shooting at the Fort Hood military installation in Texas has left at least four people dead, including the gunman, and more than a dozen injured, according to authorities.

Wednesday’s mass shooting by an Army specialist in Fort Hood, Tex., put the Pentagon on a dreaded, if increasingly familiar, footing as officials grappled to understand how yet another insider threat went undeterred.

It unfolded just two weeks after the Defense Department unveiled the findings of three investigations into last year’s fatal shooting at a Navy Yard building in Washington, D.C., by a contractor and four years after a similarly extensive inquiry into a massacre at Fort Hood by an Army psychiatrist led to vows of sweeping reforms.

Although there was little information late Wednesday about the possible motives of the gunman or the circumstances that preceded the shooting, former and current service members were shocked to learn that the Army post where Maj. Nidal Malik Hasan fatally shot 13 fellow service members in November 2009 had once again become the scene of carnage.

“We do not yet know how or why this tragedy occurred, but nearly five years after the Nidal Hasan shooting at Fort Hood in 2009, it is clear that we must do far more to ensure that our troops are safe when they are at home on base,” Rep. Thomas J. Rooney (R-Fla.), a former Army lawyer who was based at Fort Hood, said in a statement. “We must thoroughly investigate what happened today so that we can take whatever action is necessary to prevent something like this from ever occurring again.”

The shootings cast a pall over Secretary of Defense Chuck Hagel’s trip to Hawaii, where he is hosting defense ministers from Asian countries. Standing on the deck of the USS Anchorage, Hagel vowed to get to the bottom of the latest Fort Hood shooting and continue to find ways to make military installations safer.

Speaking from Chicago Wednesday, President Obama delivered remarks on the shooting at Fort Hood, saying it "reopens the pain" from the shooting there five years ago. (AP)

“We don’t have any choice here but to address what happened and do everything possible to assure the safety of our men and women who work on these bases and their families,” Hagel said.

The 2009 Fort Hood shooting and a rash of suicides on post led commanders to tighten rules on the possession of firearms by service members. Military officials have also undertaken a broader effort to improve their ability to identify troops who exhibit violent or extremist tendencies before they harm themselves or others.

In 2010, the Defense Department’s report on the Nidal shooting said officials intended to “develop a scientifically based list of behavioral indicators of potential violence.” It also said the military would work jointly with the FBI to “strengthen our understanding of the insider threat.”

That review also called for more stringent measures to vet those seeking access to military installations around the country. Due to budget constraints, some of those recommendations have not been fully implemented, according to the report the Pentagon issued last month outlining its findings of the Sept. 16, 2013, Navy Yard shooting.

The report said that several installations do not comply with physical security regulations that call for personnel screening people coming into bases to check their identification forms against military and law enforcement databases.

“Further reductions in physical security funding could put installations at risk for preventing and responding to future active shooter incidents,” the report said.

The Navy Yard review noted that officials had failed to cut off gunman Aaron Alexis’s security clearance and access to military facilities despite abundant evidence about his deteriorating mental state. Warning signs also went unheeded in the Nidal shooting and the 2010 leak of classified diplomatic cables and military reports by an Army specialist deployed to Iraq.

“Before causing such damage to the Department, all had telegraphed their personal dissatisfaction with their employers or were observed exhibiting aberrant behavior,” the Navy Yard report said. “All had legitimate access to the facilities in which they committed their offenses.”

Law enforcement officials identified the gunman in Wednesday’s shooting as Spec. Ivan Lopez, 34. Lt. Gen. Mark Milley, the top commander at Fort Hood, said that the assailant was an Iraq combat veteran who was being treated for depression and anxiety.

“Obviously, we’re digging deep into his background,” said Milley.