Congress approved the visa program in 2000 to encourage undocumented immigrants to work with law enforcement.

The first step for a U visa is local certification: Police, prosecutors or judges must certify the victim’s help in criminal cases before immigrants apply for the U visa. After being certified, the federal government scrutinizes applicants and places them on a waiting list that provides temporary residency for themselves and family members and the opportunity to apply for a work permit while they wait for their visas. It is the period to even appear on the waiting list that has ballooned.

Only 10,000 U visas are issued annually; it can take years for an immigrant to receive one, even after the criminal case they assisted has been closed. (Relatives of the immigrants are not subject to the cap.)

After this article was published, a spokeswoman for United States Citizenship and Immigration Services, Jessica Collins, said the agency has issued as many U visas as it can annually since 2010.

The average time just to be placed on a wait-list was 11 months in fiscal year 2015, according to data provided by Citizenship and Immigration Services. In 2017, more than 61,600 people applied for the visa, adding to a backlog of 190,000 pending applications.

By fiscal year 2018, the backlog had grown by more than 20 percent, to more than 229,000 pending applications, creating a wait of at least three and a half years to get placed on the waiting list. Applications for the visa declined last year for the first time, falling by more than 3,000.

Now, applicants expect to wait at least four years before receiving any protections.

In April, Kevin McAleenan, the acting homeland security secretary, told Congress that the visa program was a key part of local efforts to crack down on crime against immigrants. “The concern that the most vulnerable population in our communities are going to be victims and really, the U visa is an appropriate tool for that,” he said.