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Through the first 7 months of the program, U.S. Citizenship and Immigration Services (USCIS) approved 268,316 illegal immigrants for tentative legal status, while denying just 1,377 applications.

A Homeland Security official said the denials will tick up as time passes. Those whom the department plans to reject are given time to submit more evidence or appeal their denial, while approvals go through immediately.

For example, while USCIS approved 29,793 applications in the first six weeks of the program, it denied just six applications, or one out of every 5,000. But in March, the agency approved about 98.2 percent, meaning it denied nine out of every 500 applications.

“USCIS has issued some denials but expects denial rates to increase once requests for evidence and notice of intent to deny responses are received and reviewed by USCIS,” a Homeland Security official said.

Louis “Don” Crocetti Jr., who retired in 2011 after serving as the head of the USCIS fraud branch, also predicted his old agency’s denial rate will rise because of how it handles cases.

“It’s not uncommon, in fact it is more common than not, that the questionable cases are put on the back end in order to [make sure] the more deserving candidates get the benefit,” said Mr. Crocetti, who now runs the Immigration Integrity Group, a consultancy.

Cesar Vargas, one of those who has gained legal status under DACA and is executive political director of DRM Action Coalition, said the high approval rate makes sense given who is in this pool of immigrants.

“I am not surprised, just as most Americans and senators should not be not surprised, since many of the DACA applicants who applied were youth and students who were committed to their school and work,” Mr. Vargas said. “Dreamers have been in the U.S. for most of our lives such that it was not as difficult to put the paperwork proving our presence and moral character.”

Through the end of March, the department had received 472,004 completed applications and had settled nearly 270,000 of them.

Mr. Crocetti said DACA is a chance for the administration to test its screening process as it prepares for the possibility of a broad legalization for all 11 million illegal immigrants now in the U.S.

“We are in a post-9/11 world, as most recently evinced by the events in Boston,” he said. “This is a pivotal time that we have to get this right. We have to screen these people accurately, and we really have to know what are the key indicators to look for when these people file.”

Unlike the 1986 amnesty, when every applicant was interviewed in person and there still was double-digit fraud Mr. Crocetti said that’s not likely to be an option this time around. But he said technology has become so advanced that the agency can come up with analytical tools that can predict applications most likely to be fraudulent.

Ms. Vaughan, the policy director at the Center for Immigration Studies, which favors a crackdown on immigration, said that in the wake of the Boston Marathon bombings that should be a priority for any legalization program, including the ongoing DACA system.

“That’s very concerning in light of the most recent reminder namely this terror attack in Boston, near where I live,” she said, “that we simply are not taking enough care in screening the people we admit for legal status whether it’s this kind of deferred action or a green card or an asylum application.”