More than 1,000 international students have had their visas or legal status revoked in recent weeks, and several have filed lawsuits against the Trump administration, arguing the government denied them due process when it suddenly took away their permission to be in the U.S.

The actions by the federal government to terminate students' legal status have left hundreds of scholars at risk of detention and deportation. Their schools range from private universities like Harvard and Stanford to large public institutions like the University of Maryland and Ohio State University to some small liberal arts colleges.

At least 1,024 students at 160 colleges, universities and university systems have had their visas revoked or their legal status terminated since late March, according to an Associated Press review of university statements, correspondence with school officials and court records.

In lawsuits against the Department of Homeland Security, students have argued the government lacked justification to cancel their visa or terminate their legal status.

Why is the government canceling international students' visas?

Visas can be canceled for a number of reasons, but colleges say some students are being singled out over infractions as minor as traffic violations, including some long in the past. In some cases, students say it's unclear why they were targeted.

“The timing and uniformity of these terminations leave little question that DHS has adopted a nationwide policy, whether written or not, of mass termination of student (legal) status,” ACLU of Michigan attorneys wrote in a lawsuit on behalf of students at Wayne State University and the University of Michigan.

In New Hampshire, a federal judge last week issued a restraining order in the case of a Dartmouth College computer science student from China, Xiaotian Liu, who had his status terminated by the government. Attorneys have filed similar challenges in federal court in Georgia and California.

Homeland Security officials did not respond to a message seeking comment.

In some high-profile cases, including the detention of Columbia University activist Mahmoud Khalil, President Donald Trump's administration has argued it should be allowed to deport noncitizens over involvement in pro-Palestinian activism. But in the vast majority of visa revocations, colleges say there is no indication affected students had a role in protests.

“What you’re seeing happening with international students is really a piece of the much greater scrutiny that the Trump administration is bringing to bear on immigrants of all different categories,” said Michelle Mittelstadt, director of public affairs at the Migration Policy Institute.

How do student visas work?

Students in other countries must meet a series of requirements to obtain a student visa, usually an F-1. After gaining admission to a school in the U.S., students go through an application and interview process at a U.S. embassy or consulate abroad.

Students on an F-1 visa must show they have enough financial support for their course of study in the U.S. They have to remain in good standing with their academic program and are generally limited in their ability to work off-campus during their academic program.

Entry visas are managed by the State Department. Once they're in the U.S., international students’ legal status is overseen by the Student and Exchange Visitor Program under the Department of Homeland Security.

In recent weeks, leaders at many colleges learned the legal residency status of some of their international students had been terminated when college staff checked a database managed by Homeland Security. In the past, college officials say, legal statuses typically were updated after colleges told the government the students were no longer studying at the school.

After losing legal residency, students are told to leave the country

Historically, students who had their visas revoked were allowed to keep their legal residency status and complete their studies.

The lack of a valid entry visa only limited their ability to leave the U.S. and return, something they could reapply for with the State Department. But if a student has lost legal residency status, they risk detention by immigration authorities. Some students already have left the country, abandoning their studies to avoid being arrested.

Higher education leaders worry the arrests and visa revocations could discourage students overseas from pursuing higher education in the United States.

The lack of clarity of what is leading to revocations can create a sense of fear among students, said Sarah Spreitzer, vice president of government relations at the American Council on Education.

“The very public actions that are being taken by ICE and the Department of Homeland Security around some of these students, where they are removing these students from their homes or from their streets, that’s not usually done unless there is a security issue when a student visa is revoked,” she said. “The threat of this very quick removal is something that’s new.”

Colleges are trying to reassure students

In messages to their campuses, colleges have said they are asking the federal government for answers on what led to the terminations. Others have re-emphasized travel precautions to students, recommending they carry their passports and other immigration documents with them.

College leaders spoke of a growing sense of uncertainty and anxiety.

“These are unprecedented times, and our normal guiding principles for living in a democratic society are being challenged,” University of Massachusetts Boston Chancellor Marcelo Suarez-Orozco wrote in an email. “With the rate and depth of changes occurring, we must be thoughtful in how we best prepare, protect, and respond.”

Suarez-Orozco said the legal residency status had been canceled for two students and “five other members of our university community including recent graduates participating in training programs.”

More than 1,000 international students have had visas or legal status revoked