Before President Donald Trump took office in 2017, criminal alien incarcerations were on the decline in the U.S.

The number of convicted inmates in federal prison without citizenship plunged about 22 percent from fiscal 2011 to 2016 — before Trump was inaugurated — according to a government watchdog's study released Thursday.

That drop outpaced the total federal inmate population, which fell 8 percent in the same time period, the study reveals.

The shrinking share of criminal aliens in federal prisons appears to reflect other data showing a general downward trend in illegal immigration since its pre-recession peak in 2007.

The study shows that the trend also has led to a reduction in the government's costs associated with jailing undocumented criminals.

The annual costs of incarcerating criminal aliens, and for providing federal reimbursement to smaller governments for some of those costs fell 9 percent from fiscal 2010 through 2015 — to $1.42 billion from $1.56 billion. State prison costs also declined during the same time frame by about $60 million, to $1.1 billion per year.

The vast majority, about 77 percent, of the total number of federally incarcerated criminal aliens from 2011 to 2016 were citizens of Mexico, according to the report from the Government Accountability Office. Recent data from Pew Research show that Mexicans have routinely made up a majority of the illegal immigrant population in the U.S. as well, but their numbers fell to about half of all illegal immigrants for the first time in 2016.

The GAO's study on "criminal alien statistics" was commissioned by Republican lawmakers Sen. Chuck Grassley and Rep. Steve King of Iowa, as well as Texas Rep. Pete Sessions.

In a statement, Sessions said the report "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."

While noting that the incarceration rates have declined, Sessions said the severity of some immigrants' crimes — including human trafficking, drug smuggling and sexual assault — "proves that our national security was neglected by the previous administration."

And though illegal immigration has been on the decline, King said the report "validates President Trump's call to 'Build The Wall!'" along the U.S.-Mexico border. "Criminal aliens are exploiting our porous Southern border to gain access to our country," he added.

Neither Grassley's office nor the White House immediately responded to CNBC's request for comment.