Illegal immigrants made up a smaller share of federal prisoners in 2015 than they did five years earlier, according to a government watchdog report released Thursday.

The Government Accountability Office (GAO) reported that the federal prison system had 50,400 noncitizens in fiscal year 2011. That was about 25 percent of all prisoners. That dropped to 39,500, or about 21 percent, by Oct. 1, 2015, the start of federal fiscal year 2016.

The GAO also detected a decline in illegal immigrants in state prisons and local jails, based on a review of State Criminal Alien Assistance Program (SCAAP), a Department of Justice (DOJ) initiative that reimburses local governments part of the cost of incarcerating them.

The GAO estimated that the number at the state and local levels dropped from 282,300 in fiscal year 2010 to about 169,300 in fiscal year 2015, a 40 percent drop.

Rep. Pete Sessions (R-Texas), one of two representatives who requested the study in 2015, argued Thursday that former President Barack Obama’s policies allowed dangerous criminals to go free.

“The report released by the GAO sheds even more light on an alarming Obama-era practice that allowed criminal illegal aliens to roam our streets rather than placing them behind bars, where those who break the law belong,”

“The report released by the GAO sheds even more light on an alarming Obama-era practice that allowed criminal illegal aliens to roam our streets rather than placing them behind bars, where those who break the law belong,” he said in a statement.

“While percentages of criminal aliens in state and federal prisons decreased between 2011 and 2016, the current administration, which stringently enforces our immigration laws, has found gang members, human traffickers and drug smugglers were allowed to cross our border freely and directly into our state and country,” Sessions said.

The GAO report attributed the decline at the state and local level, in part, to a drop in the number of prisons and jails participating in the program. But the jurisdictions that did participate in the program also experienced a decline, according to the report.

David Cross, an activist who closely monitors illegal immigration and crime in Oregon, said he has noticed a similar drop in his state. He said 6.3 percent of prisoners incarcerated in Oregon last month were noncitizens. That is down from about 9 percent in 2009, he said.

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Cross, a spokesman for Oregonians for Immigration Reform (OFIR), questioned whether so-called sanctuary policies have played a role.

“My theory is some of the numbers are down, possibly, because of less cooperation from local jurisdictions,” he said.

Jessica Vaughan, director of policy studies for the Washington, D.C.-based Center for Immigration Studies (CIS), said part of the decline in noncitizens in federal prisons is due to Obama-era policies de-emphasizing immigration-related prosecutions.

Vaughan said the Obama administration severely curtailed “Operation Streamline,” an initiative similar to the zero-tolerance policy ordered by Attorney General Jeff Sessions earlier this year to target first-time border crossers for prosecution.

“Those numbers are easy to understand: A very large share of those prosecuted are incarcerated for immigration violations,” she said. “The Obama administration was not prosecuting those crimes anywhere near as aggressively as they were before.”

Immigration violations have made up 42.4 percent of the prosecutions of criminal aliens from 1974 through last year. They hit 69 percent of all noncitizen prosecutions in federal court in fiscal year 2011. But that number tumbled 34 percent, falling from 27,912 in fiscal year 2011 to 18,359 in fiscal year 2016.

The share of immigration violations fell from 69 percent of noncitizen crime to 65 percent in fiscal year 2016.

But the number of noncitizens prosecuted in federal court fell for most other categories of crime as well. The 30 percent overall decline in prosecutions was three times greater than the drop in federal prosecutions of American citizens.

Rep. Steve King (R-Iowa), the other congressman who requested the study, contended that the report points to how “reckless and lax” the Obama administration was.

Related: Illegal Immigration Now Far More Dangerous Than Five Years Ago

“Further, it validates President [Donald] Trump’s call to ‘Build The Wall!’ along our border with Mexico,” he said in a statement. “Over 90 percent of criminal aliens in federal prisons are citizens of just six countries: Mexico, Honduras, El Salvador, Columbia, Guatemala, and the Dominican Republic. The location of these countries leaves little doubt that criminal aliens are exploiting our porous border to gain access to our country.”

The GAO analysis found that the decline in noncitizens behind bars saved taxpayers money. Incarceration costs for the federal system and federal reimbursement of state and local facilities declined from $1.56 billion in fiscal year 2010 to $1.42 billion in fiscal year 2015.

The expense of correctional officer salaries, medical care, food service, utilities, and other costs associated with operating the SCAAP program also dropped, declining from $1.17 billion to about $1.11 billion from fiscal years 2010 to 2015.

Vaughan, however, said it is not clear that taxpayers saved money overall. She said when illegal immigrants flood communities, they put a strain on school and municipal budgets. Noncitizens who commit crimes divert police resources that could be put into other law enforcement activities, she said.

“There are a lot of other costs that go along with this that take a toll on local budgets,” she said.