NATIONAL NEWS (CNN) — It's a scene that's becoming all too familiar at Fort Hood: flag-draped caskets for soldiers who've survived war, only to be killed at home.
A shooting Wednesday left four people dead, including the gunman, who committed suicide. It also sent ripples of fear across a military installation still on edge following a previous massacre.
Maj. Nidal Hasan killed 13 people and injured dozens in November 2009.
Two years later, an Army private tried to detonate explosives at fellow soldiers from the same facility -- but was foiled.
"Tonight, Texans' hearts are once again very heavy. The scenes coming from Fort Hood today are sadly too familiar and still too fresh in our memories," U.S. Sen. John Cornyn said.
What is it about Fort Hood that makes it a target for soldier-on-soldier attacks? Is it more dangerous? More vulnerable than most?
Not so, experts say.
"First of all, when it comes to a military base, this is where a nation's war fighters are being trained. So, by nature, they're being prepared for combat or they've returned from combat zones around the world," said Thomas Fuentes, a former FBI assistant director.
"So they come back and you're asking people to turn that switch on or off," he said. "To now return to a peaceful civilian life -- you know get along with your neighbors and your families -- that's often easier said than done with some people."
The large Fort Hood population makes it susceptible to gun violence, just like any other city in the U.S., Fuentes said. Fort Hood is the nation's largest Army post.
"You're talking about a small city with a population of about 50,000 people," he said. "So there are acts of violence that occur in cities and towns ... by sheer population. You hope it doesn't occur at a controlled environment like a military base, but unfortunately it does."
Changes after 2009
In the wake of the 2009 massacre, authorities launched various investigations and reviews.
"The tragic shooting of U.S. military personnel at Fort Hood in November 2009 underscored the need for the Department of Defense to thoroughly review its approach to force protection," Defense Secretary Robert Gates wrote in a memo shortly after that shooting.
Authorities put in place a series of changes.
They included creating a kind of "neighborhood watch" to teach the Army community to recognize and report suspicious behavior.
Gates also called for a plan to educate commanders on signs of workplace violence and an enhanced 911 systems on military installations to enable emergency dispatchers to effectively determine a caller's location.
Other recommendations included upgraded health care to service members and the hiring of additional providers especially in mental health to help meet that goal.
The Army must transform how it protects its soldiers, collects information about internal threats and communicates with the FBI and terrorism experts, investigators said at the time.
The changes were crucial. But in this case, at least, it seems they failed.
Authorities say Spc. Ivan Lopez opened fire at Fort Hood on Wednesday, killing three people before fatally shooting himself. Sixteen more people were injured.
The suspect, a soldier who served for four months in Iraq, "had behavioral health and mental health" issues, said Lt. Gen Mark Milley, the post's commanding general.
He suffered from depression, anxiety and other psychiatric complaints and was receiving treatment and medication.
He was going through the process required to diagnose Post-Traumatic Stress Disorder (PTSD).
"He was not diagnosed, as of today, with PTSD," Milley said.
Lopez was transferred to Fort Hood from another unnamed base in February. Retired Army Maj. Gen. James "Spider" Marks, a CNN analyst, expressed surprise at the transfer.
Lopez should have remained at the other base for continuity of care, he said.
"We will have to reexamine all of those programs to see if there were any gaps," Milley said.
Gun on base
Lopez used a .45 caliber Smith and Wesson pistol that he recently purchased in the area, but did not register it on base.
A ban enacted in 1993 prohibits soldiers from carrying guns on military bases.
They have to keep them locked in the base armory, and have to get special permission to take them out.
In September, House Republicans introduced legislation to repeal the ban. Texas Rep. Steve Stockman was one of seven Republicans who proposed the Safe Military Bases Act, saying the bill would help thwart mass shooting on bases.
Authorities will look at what, if anything, they could have done differently.
That might be an uphill challenge.
"Fort Hood, tens of thousands of people come and go from that every day," said CNN's Pentagon correspondent Barbara Starr. "They have stickers in their cars; they have permits to get on base. They are waived through a gate, because traffic has to keep flowing."
"If they are permitted onto the base because they have the right paperwork on any given day. It is not likely that their cars will be searched every time they go in and out. It is assumed that they are acting appropriately and acting legally. That's just the way the reality is."
Lopez and Hasan's attacks weren't the only ones at the base where soldiers were targets.
In 2011, Army Private Naser Jason Abdo was arrested after he tried to build a bomb to blow up Fort Hood troops.
A search of his hotel room revealed various items used to make bombs, including battery operated clocks and a pressure cooker. He had refused to deploy to Afghanistan and later disappeared from Fort Campbell, Kentucky, after being charged with possession of child pornography. He planned to bomb a Killeen restaurant filled with soldiers. Fort Hood is in Killeen.
A federal judge sentenced him to life in prison.
Lessons of resilience
"Events in the past have taught us many things here at Fort Hood," Milley said.
"We know the community is strong and resilient, and we know the soldiers and civilians of the families of this fort who have served so bravely in combat over the last 13 years in Iraq and Afghanistan are strong and we will get through this."
And as the community mourns, President Barack Obama said he's heartbroken about the repeated attack.
"Obviously this reopens the pain of what happened at Fort Hood five years ago," Obama said.
After battling enemies at war zones, U.S. troops should feel safe at home, he said.