"We remain committed to closing Guantanamo because it's in our national security interest," White House spokesman Jay Carney told reporters.
Such an assessment is what "military commanders have said and previous administrations said prior to the arrival and the inauguration of this president," Carney said.
"Obviously that's a process that is ongoing," he said.
President Barack Obama's administration said Monday it would try Khalid Sheikh Mohammed and four other alleged 9/11 plotters at the US military base in Cuba, a major about-face after intense criticism in Congress on a plan for a civilian trial.
Obama pledged on taking office in 2009 to shut down Guantanamo -- seen by critics as a symbol of excesses in former president George W. Bush's "war on terror" -- within a year.
But Obama has failed to meet the deadline due largely to pressure from the US Congress, with lawmakers staunchly opposing efforts to bring most-wanted extremists onto US soil for trial.
Attorney General Eric Holder had initially hoped to try Mohammed, a Kuwaiti-born Pakistani, at a civilian court in New York City near the site of the devastating 2001 attack in a show of openness for the US judicial system.