Federal officials on Tuesday announced they completed what they called a "first-of-its kind" operation earlier this week targeting immigration fraud in the Minneapolis and St. Paul metro area, leading to four people in custody.

U.S. Citizenship and Immigration Services, in coordination with the Federal Bureau of Investigation and U.S. Customs and Immigration Enforcement, conducted "Operation Twin Shield" over nine days and looked at more than 1,000 cases for "fraud or ineligibility indicators," said Joseph Edlow, director of USCIS.

That yielded 275 cases of "suspected fraud." But it narrowed to 42 case referrals to ICE and just four arrests so far. Nobody has been charged. 

"This is what the government should be doing," Edlow told reporters Tuesday in a press conference. "We have a responsibility to keep this system legitimate, and when you see what has been uncovered by these diligent officers over the past two weeks, less than two-week period, that's only the tip of the iceberg to what we're actually dealing with when it comes to immigration fraud."

Edlow cited examples of what federal officers uncovered during the investigation like sham marriages, including one where a person took advantage of an elderly U.S. citizen. Another case involved a man admitting to fabricating a death certificate to falsely claim the end of a marriage, though the spouse who is the mother of his five children is alive in Minneapolis.

"What they found should shock all of America," Edlow said. 

Ana Pottratz Acosta, a visiting professor at the University of Minnesota Law School teaching at the Immigration and Human Rights Clinic, said immigration fraud does happen, but it's rare. 

She believes the numbers released by federal officials on Tuesday underscore that. 

"There were 275 cases where there was some sort of flag raised during those investigations, but only 42 cases out of approximately 900 or 1,000 where they commenced deportation or removal proceedings, and only four of them were arrested," Pottratz Acosta said. "So if you look at the actual numbers, 42 cases out of 900 or 1,000, that's less than 5%. And four cases out of 1,000, that's like half a percent, so it's a very small number and they're completely blowing it out of proportion."

When asked why the Twin Cities was the first city for such an operation, Edlow pointed to data showing "concerning" patterns of fraud, but declined to share more specifics. 

The announcement comes one day after the U.S. Department of Justice said it filed a lawsuit against the cities of Minneapolis and Saint Paul for their so-called "sanctuary city" policies. 

Pottratz Acosta added that there is a presumption "baked into our immigration laws" that a person getting married to a U.S. citizen or pursuing that relationship is doing so solely for immigration purposes so there is a high burden on the applicant to prove it's legitimate. 

Similarly, there are strict requirements for H-1B visas for foreign workers and student visas, she said.

"Our immigration system is very unforgiving, so even if you make a mistake, it's often hard to fix it or correct the record, which is why it's very important to be very careful before anything even gets submitted," she said. "And I think it also is reflected in the very small number of notices to appear and arrests that resulted from an investigation of what they say is over 1,000 cases."

USCIS said the agency expects ICE referrals to increase as more investigations stemming from this operation are completed. The operation began Sept. 19 and ended Sunday.

Edlow added that the Twin Cities is just the first area to be targeted in this way, though he did not say what other cities may be next on the list.

"I would say any city should be prepared to be the next site for an operation of this magnitude," he said.