Working a Saturday shift in the stuffy Immigration and Naturalization Service office in downtown Los Angeles in the 1970s, Carl Shusterman came across a rap sheet.

A man recently sworn in as a United States citizen had failed to disclose on his naturalization application that he had been arrested, but not convicted, in California on rape and theft charges.

Shusterman, then a naturalization attorney, embarked on a months-long effort to do something that rarely happened: strip someone of their American citizenship.

“We had to look it up to find out how to do this,” he said. “We’d never even heard of it.”

Forty years later, denaturalization — a complex process once primarily reserved for Nazi war criminals and human rights violators — is on the rise under the Trump administration.

A United States Citizenship and Immigration Services team in Los Angeles has been reviewing more than 2,500 naturalization files for possible denaturalization, focusing on identity fraud and willful misrepresentation. More than 100 cases have been referred to the Department of Justice for possible action.

“We’re receiving cases where [Immigration and Customs Enforcement] believes there is fraud, where our systems have identified that individuals used more than one identity, sometimes more than two or three identities,” said Dan Renaud, the associate director for field operations at the citizenship agency. “Those are the cases we’re pursuing.”

The move comes at a time when Trump and top advisors have made it clear that they want to dramatically reduce immigration, both illegal and legal.

The administration granted fewer visas and accepted fewer refugees in 2017 than in previous years.

Recently, the federal government moved to block victims of gang violence and domestic abuse from claiming asylum. White House senior advisor Stephen Miller — an immigration hawk — is pushing a policy that could make it more difficult for those who have received public benefits, including Obamacare, to become citizens or green card holders, according to multiple news outlets.

Shusterman, now a private immigration attorney in L.A., said he’s concerned denaturalization could be used as another tool to achieve the president’s goals.

“I think they’ll … find people with very minor transgressions,” he said, “and they’ll take away their citizenship.”

Dozens of U.S. mayors, including L.A.’s Eric Garcetti, signed a letter sent to the citizenship agency’s director in late July, criticizing a backlog in naturalization applications and the agency’s commitment of resources to “stripping citizenship from naturalized Americans.”

“The new measure to investigate thousands of cases from almost 30 years ago, under the pretext of the incredibly minimal problem of fraud in citizenship applications, instead of managing resources in a manner that processes the backlogs before them, suggests that the agency is more interested in following an aggressive political agenda rather than its own mission,” the letter stated.

But Mark Krikorian, executive director of the Center for Immigration Studies, which supports tighter controls, said “denaturalization, like deportation, is an essential tool to use against those who break the rules.”

“It’s for people who are fraudsters, liars,” he said. “We’ve been lax about this for a long time, and this unit that’s been developed is really just a question of taking the law seriously.”

From 2009 to 2016, an average of 16 civil denaturalization cases were filed each year, Department of Justice data show. Last year, more than 25 cases were filed. Through mid-July of this year, the Justice Department has filed 20 more.

Separately, ICE has a pending budget request for $207.6 million to hire 300 agents to help root out citizenship fraud, as well as to “complement work site enforcement, visa overstay investigations, forensic document examination, outreach programs and other activities,” according to the agency.

The stage for increasing cases of denaturalization was set during the waning days of the Obama administration.

In September 2016, a report released by the inspector general for the Department of Homeland Security showed that 315,000 old fingerprint records for immigrants who either had criminal convictions or deportation orders against them had not been uploaded into a database used to check identities.

It turned out that because of incomplete fingerprint records, citizenship had been granted to at least 858 people who had been ordered deported or removed under another identity. USCIS began looking into cases.

John Sandweg, who headed U.S. Immigration and Customs Enforcement under Obama, said that when it came to denaturalization, officers considered it on a case-by-case basis, “looking at the seriousness of the offense and then deciding if it made sense to dedicate the resources.”

“It was looked at more in that context — let’s look for serious felons who may have duped the system because we didn’t digitize fingerprints yet. Not so much … let’s just find people where there’s eligibilities to denaturalize because we want to try to reduce the ranks of naturalized U.S. citizens.”

Even during the communist scare of McCarthy era, citizenship revocation was so rare that often the cases made the news.

“The constant surveillance of communists in this country is a 24-hour, seven-days-a-week, 52-weeks-a-year job,” President Eisenhower declared in 1954, according to a Los Angeles Times article headlined: “Eisenhower cites U.S. war on reds.”

The government in 1981 took citizenship away from Feodor Fedorenko, who had worked as a guard at a Poland death camp, fled to the U.S. and illegally obtained citizenship by omitting references to his Nazi service. After he was denaturalized, he was deported to the Soviet Union and executed as a war criminal.

“It’s always taken expertise and finesse to bring those cases to court and successfully finish,” said Bruce J. Einhorn, former litigation chief for Justice Department’s Office of Special Investigations. “I think an office like this, in theory, could do a great deal of good, depending also on their exercise of prosecutorial discretion.”

Citizenship and Immigration Services began training officers last year on how to review cases and on the burden of proof necessary to revoke a person’s citizenship. About a dozen people are in the L.A. unit — a number expected to rise to about 85 with the addition of support, analyst and administrative staff.

The case of Baljinder Singh, of India, is among those the agency referred to Justice officials.

Nearly three decades ago, Singh arrived in San Francisco from India without any travel documents or proof of identity, claiming his name was Davinder Singh. He was placed in exclusion proceedings but failed to show up for an immigration court hearing and was ordered deported.

He later filed an asylum application under his true name but withdrew it after he married a U.S. citizen who filed a visa petition on his behalf, according to the Justice Department. He became a citizen on July 28, 2006.

In January, a federal district judge revoked Singh’s citizenship.

“I think that if individuals saw these cases and really took time to understand the length to which some of these individuals went to fraudulently obtain immigration status, they too would want us to pursue these cases,” Renaud said.

Einhorn said that what many view as the Trump administration’s anti-immigration agenda makes it hard to see denaturalization and the citizenship agency’s role in it in a neutral way.