A group of 13 members of the Senate Democratic caucus, led by Massachusetts’ Elizabeth Warren, are expressing new concerns about a decision not to provide seasonal flu vaccines to migrants in U.S. detention.

“This dangerous decision not to administer vaccinations for a disease that has already proven fatal to migrant children in CBP’s custody is immoral and irresponsible, placing entire communities at risk of the flu and its associated complications,” the senators wrote to acting Homeland Security Secretary Kevin McAleenan and Health and Human Services Secretary Alexander Azar.

Aside from the specific questions about vaccination procedures, the senators are seeking details of what quarantine procedures may be in place.

“When CBP detains migrant children and adults, what screening and testing processes are in place to identify potential influenza-like illness or other health risks? Who is conducting the screening and providing any needed treatment?” the senators asked, also asking for some specific accounting. “How many pediatricians does CBP contract with? Are these pediatricians on site at DHS facilities, and if so, which ones?”

Joining Warren on the letter, which was dated Friday and released publicly Monday, are Richard Blumenthal of Connecticut, Amy Klobuchar of Minnesota, Cory Booker of New Jersey, Kamala Harris of California, Jack Reed of Rhode Island, Patty Murray of Washington, Thomas R. Carper of Delaware, Mazie Hirono of Hawaii, Sherrod Brown of Ohio and Jeff Merkley and Ron Wyden, both of Oregon. 

Bernie Sanders, a Vermont independent who caucuses with the Democrats, also signed on.

Booker, Klobuchar, Harris, Sanders and Warren are seeking the Democratic nomination for president.

The senators are seeking responses by Sept. 20.

DHS and CBP did not immediately respond to a request for comment.