Massachusetts Sens. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (D) and Ed Markey Edward (Ed) John MarkeyDemocratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy McConnell says Trump nominee to replace Ginsburg will get Senate vote Massachusetts town clerk resigns after delays to primary vote count MORE (D) expressed “grave concerns” in a letter Sunday about reported plans to deploy Customs and Border Protection (CBP) SWAT teams into so-called sanctuary cities.

In the letter to the heads of CBP, Immigration and Customs Enforcement (ICE) and the Department of Homeland Security (DHS), the two addressed the CBP announcement that its Border Patrol Tactical Unit (BORTAC) will be deployed to several cities that refuse to cooperate with the Trump administration on immigration enforcement.

"The BORTAC deployment to Boston and other cities is unnecessary, unwelcome, and dangerous. But the specter of heavily armed, military-like personnel in our cities will accomplish one thing: provoke fear," Warren and Markey wrote.

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"We write to demand that you reverse course and to pose questions to better understand your rationale for employing paramilitary-style immigration personnel...in Boston and elsewhere in the United States,” they added.

In the latter, Warren and Markey called for the department heads to describe how many of the 100 CBP personnel deployed to assist ICE are BORTAC members, what communities BORTAC will be deployed in, what specific orders the teams have been given, and how many BORTAC agents have been deployed to Boston.

The two also asked for answers about why the deployment was necessary in American cities and what evidence of threats supports the decision, as well as whether the deployment is motivated by a desire to retaliate against opponents of the White House’s immigration policies, and whether it will affect readiness at the U.S.’ southern border.

The deployment, set to run from February through May, reportedly targets cities including San Francisco, Los Angeles, Atlanta, Houston, New Orleans, Detroit and Newark, New Jersey.