BOSTON (CBS) – In Boston, Attorney General Jeff Sessions announced the arrests of more than 20 people charged with immigration document and benefit fraud on Thursday.

Sessions stressed that the focus of the probe, dubbed “Operation Double Trouble,” was on undocumented immigrants defrauding the government and taxpayers in Massachusetts.

“We are an open, generous nation. Accepting illegal immigration, however, would be a disservice to the legal immigrants who played by the rules, waited their turn, respected our laws, our customs and our way of life,” Sessions said.

“You do not get to come to America unlawfully. Let’s just make that clear,” he said.

While in Boston, Sessions defended Assistant Attorney General Rod Rosenstein, after 11 GOP lawmakers introduced articles of impeachment against the Justice Department official who oversees special counsel Robert Mueller’s Russia investigation.

The Task Force assigned to Operation Double Trouble reviewed Medicaid billing data and found more than 110 cases of people with the same name and the same social security numbers.

“Two people, same name, same numbers. One in Puerto Rico, and one in Massachusetts receiving Medicaid benefits,” Sessions said. “In some cases, both people with the same identity were receiving medical services on the same day, in both jurisdictions, 1,600 miles apart.”

The alleged fraudsters are primarily Dominican nationals residing in Massachusetts, including in Haverhill, Roslindale, Malden, Lawrence, Salem, Brockton, Revere, Dorchester, Lynn, and Mattapan. Their alleged victims are predominantly American citizens in Puerto Rico, Sessions said.

The defendants included a convicted drug trafficker, who was receiving unemployment benefits and living in taxpayer-funded housing in Massachusetts, and who allegedly stole the identity of an American citizen from Puerto Rico who was displaced by Hurricane Maria.

Identity theft is a common offense committed by undocumented immigrants, Sessions said.

“With few exceptions, illegal aliens cannot have social security numbers, and so in many cases they steal them, usually so they can take jobs they are not entitled to or to receive other benefits,” Sessions said. “But some illegal aliens steal identities to get money from the government.”

Federal law prohibits undocumented immigrants from receiving most cash benefits, and they are ineligible for programs like Medicare and Social Security.

“And so many of them lie, to defraud the taxpayers of this country including the hardworking people of Massachusetts,” Sessions said.

The attorney general said an estimated 180,000 people in greater Boston are in the country illegally, ranking it among the top 12 metro areas in the U.S. in terms of undocumented immigrant populations.

“We’re talking about illegal immigration, not legal immigration,” Sessions said. “We take in 1.1 million people each year to legal permanent residence with a guaranteed pathway to citizenship. Another 700,000 come to our country every year on work visas. And, over a half-million come to take spots in colleges and universities.”

Sessions joined U.S. Attorney Andrew E. Lelling for a 10 a.m. press conference at the Moakley Federal Courthouse in Boston.

Also attending were Peter C. Fitzhugh, special agent in charge for Homeland Security investigations in Boston and Phillip M. Coyne, special agent in charge for the U.S. Department of Health and Human Services, Office of the Inspector General.