Located in Killeen, Texas, Fort Hood is the largest U.S. military facility in the world and the site of the largest armored training centers.
Spanning 340 square miles, Fort Hood is located roughly 60 miles north of the Texas capitol of Austin and 50 miles south of Waco and is home to more than 5,000 military families.
Fort Hood has lost more troops in the war in Iraq than any other U.S. based military facitlity, and troops have seen some of the fiercest fighting there. Since the war's inception, 483 Fort Hood soldiers have died there. Currently, about 15,000 troops from the fort are deployed in Iraq and the 1st Cavalry Division is in its third deployment. More than 20 Fort Hood soldiers have been killed in Afghanistan since 2001.
Army Spc. Jonathan Pacheco told ABCNews.com that almost everyone at Fort Hood is acutely aware that they had taken a larger hit in terms of overseas casualties than any other base in the United States, but that the mood had been relaxed recently since the conflict in Iraq seemed to be "stepping down."
In an interview with ABC News in April 2006, Sgt. Todd Singleton said of the experience, "There's no way to forget it. You see things you should never see in life."
It was the 4th Infantry Division from Fort Hood that captured ousted Iraqi President Saddam Hussein.
Today's shooting is also not the first such incident for this city. In September 2008, 22-year-old Spc. Jody Michael Wirawan shot and killed another officer. When police arrived at the scene, Wirawan shot and killed himself.
Killeen was also home to the deadliest shooting rampage in modern U.S. history until the Virginia Tech massacre. On Oct. 16, 1991, 35-year-old George Jo Hennard drove his pickup truck through the window of a Luby's Cafeteria and fatally shot 23 people and injured 20 more before turning the gun on himself.
"These are people who are already under pressure. Fort Hood...has really borne the brunt of the war on terror," Texas Republican Sen. Kay Bailey Hutchison told ABC News today. "They have families who have stood beside them as they are fighting for America, and to be shot down in cold blood like this is something you never think will happen in America."
The city of Killeen was formed in 1882 when the first railroad was built there by the Santa Fe Railway Company. Named after Frank P. Killeen, the rural city soon became a small farming and ranching town. But the city didn't expand until the creation of the U.S. base in the 20th century.
In 1942, Camp Hood was created and was named in honor of famous Confederate Army Gen. John Bell Hood, who gained notoriety during the Civil War. In 1950, it was redesignated as a fort and in that decade, the size and scope of the facilities were expanded as the Cold War gained pace.
In 1953 alone, nearly 400 officer quarters and about 200 family housing units were built, in addition to an elementary school, a water supply system and a post exchange, according to Global Security. Today, there are more than 5,000 sets of quarters for soldiers and their families and about 634 quarters for officers and their family members.
According to the most recent census, the population of Killeen is 116,934, most of which are troops at Fort Hood and their families.
"Our families have really sacrificed much more than any of us could have, and it's really hard to -- thanks ain't enough, but it's all we can say to them," Col. Robert Adams told ABC News' Martha Raddatz in June 2005.
Spouses of troops deployed in Iraq say they all share a bond.
"It creates a bond. They know your experience. They share that sense of loss, and the grieving, and they don't tell you to just move on," Tracy Cunningham told ABC News in May, 2005. "You can't just move on. Everybody here at Fort Hood will say, 'if you need something, call us.' And they meant it. They take care of you."
The Fort Hood's Darnall Army Hospital, where the suspect in today's shooting is expected to have been stationed, opened its door in April 1965.
The 1st Cavalry Division can trace its history back to the 1850's, when it engaged in battles against Native Americans.
ABC News' Martha Raddatz, Lisa Chinn and Sarah Netter contributed to this report.