This 2016 photo shows an unused guard tower at Camp Delta on the U.S. Naval base at Guantanamo Bay, Cuba. (Ben Fox/AP)

On his last day in office, President Obama repeated an eight-year request to Congress: Close the military prison in Guantanamo Bay, Cuba.

“There is simply no justification beyond politics for the Congress’ insistence on keeping the facility open,” Obama wrote in a letter to Speaker of the House Paul D. Ryan that was released by the White House on Thursday. “Members of Congress who obstruct efforts to close the facility, given the stakes involved for our security, have abdicated their responsibility to the American people.”

In his first inaugural address in 2009, Obama said that the detention center would shutter during his tenure. He has long said the facility “undermines national security” by serving as a source of propaganda for terrorists and a drain on military resources. Now that Obama is leaving, the prison remains open for an incoming president who has vowed to continue using it.

During his campaign, Donald Trump said that he would fill the facility with some “bad dudes,” and he even suggested trying U.S. citizens there, which is prohibited under U.S. law. The last time the United States added to the prison population at Guantanamo was before Obama took office.

Opened in the months after the Sept. 11, 2001, attacks to hold captured terrorism suspects, Guantanamo reached a peak population of more than 700 detainees during George W. Bush’s administration. Under President Obama, roughly 196 of them have been resettled or relocated. Now, according to Obama’s letter, only 41 remain.

In December, the White House told Congress that it would transfer 19 detainees before Obama left office. Earlier this week, the Pentagon said it was moving a portion of those to Saudi Arabia and the United Arab Emirates.

Despite Obama’s attempt to whittle down the population at the prison, a core group of detainees remain. Those include some prisoners whom officials say they cannot release because of security risks but who also cannot be tried because the government does not have enough evidence. The group includes self-proclaimed Sept. 11 mastermind Khalid Sheikh Mohammed, whose trials likely won’t end for another three years.

Obama has argued that Republicans stopped him from shutting down the facility, while his critics say the president could have done more. In 2015, the Obama administration lobbied to move prisoners from Guantanamo to facilities in the United States in order to close the prison once and for all. The proposal garnered swift backlash from Republicans and tighter restrictions for prisoner resettlement.

“History will cast a harsh judgment on this aspect of our fight against terrorism and those of us who fail to bring it to a responsible end,” Obama wrote. “Guantanamo is contrary to our values and undermines our standing in the world, and it is long past time to end this chapter in our history.”