Forty-four Afghan troops visiting the United States for military training have gone missing in less than two years, presumably in an effort to live and work illegally in America, Pentagon officials said.

Although the number of disappearances is relatively small — some 2,200 Afghan troops have received military training in the United States since 2007 — the incidents raise questions about security and screening procedures for the programs.

They are also potentially embarrassing for U.S. President Barack Obama’s administration, which has spent billions of dollars training Afghan troops as Washington seeks to extricate itself from the costly, 15-year-old war. The disclosure could fuel criticism by supporters of Republican presidential nominee Donald Trump, who has accused the Obama administration of failing to properly vet immigrants from Muslim-majority countries and has pledged a much tougher stance if he wins.

While other foreign troops on U.S. military training visits have sometimes run away, a U.S. defense official said that the frequency of Afghan troops going missing was concerning and “out of the ordinary.”

Since September alone, eight Afghan troops have left military bases without authorization, Pentagon spokesman Adam Stump told Reuters. He said the total number of Afghan troops who have gone missing since January 2015 is 44, a number that has not previously been disclosed.

“The Defense Department is assessing ways to strengthen eligibility criteria for training in ways that will reduce the likelihood of an individual Afghan willingly absconding from training in the U.S. and going AWOL (absent without leave),” Stump said.

Afghans in the U.S. training program are vetted to ensure they have not participated in human rights abuses and are not affiliated with militant groups before being allowed into the United States, Stump said.

The defense official, who spoke on condition of anonymity, added there was no evidence any of those who had absconded had carried out crimes or posed a threat to the United States.

The Afghan army has occasionally been infiltrated by Taliban militants who have carried out attacks on Afghan and U.S. troops, but such incidents have become less frequent due to tougher security measures.

Trump, whose other signature immigration plan is to build a wall on the U.S.-Mexico border, has proposed a temporary ban on Muslims seeking to enter the country, and has said that law enforcement officers should engage in more racial profiling to curb the threat of attacks on American soil.

After Omar Mateen, whose father was born in Afghanistan, killed 49 people at a gay nightclub in Orlando in June, Trump said an immigration ban would last until “we are in a position to properly screen these people coming into our country.”