Federal immigration officials in Boston are releasing more details on what specially trained U.S. Customs and Border Protection (CBP) agents and officers will be doing in and around the city.

The border officials were deployed to Boston to assist ICE agents with arresting undocumented immigrants that have criminal backgrounds, according to Todd Lyons, the acting director of U.S. Immigration and Customs Enforcement's (ICE) Boston Field Office.

Lyons points to so-called sanctuary city policies, including Boston's Trust Act and a 2017 Supreme Judicial Court decision, when justifying the need for reinforcements from the border agency.

"Just because of the amount of cases, with the Trust Act, with the Lunn decision, where we're having more criminal aliens released, we just don't have the infrastructure or the manpower to go after those criminal targets," he said in an interview Wednesday.

According to Lyons, "criminal alien" is a broad term used by the agency to describe anyone in the country without documented immigration status and have criminal charges or convictions.

Boston and many other Massachusetts communities have established so-called sanctuary policies in order to limit interaction between local law enforcement and federal immigration officials. The state's highest court also ruled in Commonwealth v. Lunn that local law enforcement officials cannot hold an individual who otherwise would be free to go solely at the request of ICE. These requests are made via administrative warrants known as ICE detainers.

Immigrant advocates fear ramped-up enforcement will further intimidate some individuals from reporting crimes, making communities less safe. Eva Millona, the executive director of the Massachusetts Immigrant and Refugee Advocacy Coalition (MIRA), denounced the presence of tactical CBP units in Boston.

"We hope that this CBP deployment is mostly a publicity stunt, and we won't see aggressive, paramilitary operations in our communities, which would be dangerous and deeply traumatic," Millona said in a statement.

On Wednesday, Lyons attempted to allay such fears, saying the public will not see specialized weapons or tanks being used for immigration arrests.

"There won't be tactical SWAT teams patrolling the streets," he said. "These are specifically trained officers that are a force multiplier."

The U.S. Department of Homeland Security says sanctuary city policies result in less cooperation with local law enforcement, meaning ICE is less likely to make immigration arrests of people with criminal backgrounds in secure places like courtrooms and jails. Instead, more arrests are happening in communities, which ICE says puts everyone at greater risk.

Lyons welcomed the reinforcements from the border who come with special skills that he said will be of use in making arrests in the field.

"High-risk vehicle stops, clearing buildings, so those are the type of officers that are being deployed, they're used to dealing in an urban environment," he said.

In conjunction with the additional CBP resources, the Boston Field Office of ICE is now releasing the names, criminal histories and photos of individuals who the agency says were released by local law enforcement and court officials after an ICE detainer request was denied. Some of the charges include child rape, domestic violence and multiple driving under the influence infractions, though not all of the criminal histories mention actual criminal convictions.

Lyons said two arrests were made shortly after the release of the identifying information on Friday. He said tips were used in locating two of the people who he referred to as "immigration fugitives."

Senators Elizabeth Warren and Ed Markey on Sunday sent a letter to federal immigration officials calling the deployment of specially trained border officials an unnecessary escalation and demanding the Trump administration withdraw the plan.

Correction: A previous version of this story mischaracterized ICE's use of the term "criminal alien." The story has been updated.