U.S. Sens. Elizabeth Warren and Ed Markey, D-Massachusetts urged the Trump administration Monday to provide a list of all the migrant children and parents separated at the United States' southern border under the White House's "zero tolerance" approach to immigration.

The Massachusetts Democrats joined other senators in asking Departments of Health and Human Services and Homeland Security officials for anonymized, de-identified information by July 6 on all of the children and adults affected by the controversial policy.

Stressing that they are "deeply concerned by reports of chaotic attempts to reunify parents and children that have been separated at the border," the lawmakers sent a letter to HHS Secretary Alex Azar and DHS Secretary Kirstjen Nielsen requesting the status of efforts to reunite migrant families -- particularly in the wake of recent executive action.

"Since the president's executive order was signed on June 20, 2018, the administration has repeatedly indicated that families would be reunited, claiming that 'Department of Homeland Security and Health and Human Services have a process established to ensure that family members know the location of their children and have regular communication after separation' ... However, the hastily signed order provided no clarity on how to reunify families or how to handle families that have already been separated or new families that cross the border seeking asylum," they wrote.

The senators further raised concerns that "even as the administration works to reunify families, it continues to deport adults and family members who had children taken from them -- reducing their chances of reunification even further."

The lawmakers requested information on how long migrant parents and children have been separated, whether the Office of Refugee Resettlement has identified and/or contacted those from whom they were separated and whether the separated children have been released to a sponsor or deported, among other things.

Senators also asked that DHS and HHS officials brief the Senate on the administration's progress in reuniting families.

Beyond the Massachusetts Democrats, U.S. Sens. Bill Nelson, D-Florida; Bernie Sanders, I-Vermont; Bob Casey, D-Pennsylvania; Kirsten Gillibrand, D-New York; Richard Blumenthal, D-Connecticut; Mazie Hirono, D-Hawaii; Cory Booker, D-New Jersey; Tammy Duckworth, D-Illinois; and Tina Smith, D-Minnesota, also signed the letter.

Warren and Markey, who have vocally criticized the Trump administration's immigration policies, both recently visited the U.S.-Mexico border to tour detention facilities.

President Donald Trump signed an executive order last month to stop federal officials from separating families who entered the United States illegally.

The president said the executive order will "keep the families together" while ensuring that the United States' strong borders. He, however, stressed that his administration will still take a "zero tolerance" approach to immigrants who enter the U.S. illegally.

A federal judge, meanwhile, ordered federal officials to quickly reunite migrant children who were separated from their parents.

The Trump administration, in turn, announced Friday that it would hold migrant families together in detention, various news outlets reported.