WASHINGTON — Lawyers working to reunite migrant families separated by the Trump administration before and during its “zero tolerance” policy at the border now believe the number of separated children for whom they have not been able to find parents is 666, higher than they told a federal judge last month, according to an email obtained by NBC News.

Nearly 20 percent of those children were under 5 at the time of the separation, according to a source familiar with the data.

In the email, Steven Herzog, the attorney leading efforts to reunite the families, explains that the number is higher because the new group includes those “for whom the government did not provide any phone number.” Previously, the lawyers said they could not find the parents of 545 children after they had tried to make contact but had been unsuccessful.

Herzog said in the email to Justice Department attorneys representing the Trump administration, “we would appreciate the government providing any available updated contact information or other information that may be helpful in establishing contact for all 666 of these parents.”

Lee Gelernt, deputy director of the ACLU Immigrants’ Rights Project, explained to NBC News that the new number “includes individuals in addition to 545 for whom we got no information from the government that would allow meaningful searches but are hopeful the government will now provide with that information.”

The Trump administration imposed a “zero tolerance” policy on the U.S./Mexico border between April and June 2018 under which undocumented migrant parents and children were separated. Prior to the border wide “zero tolerance” policy, the Trump administration tested family separation in a pilot program in the El Paso sector. The vast majority of the children referenced in the email obtained by NBC News were separated during this pilot program, but the total also includes some children who were separated under zero tolerance.

President-elect Joe Biden has committed to establishing a government task force that would work to reunite all migrant families separated by Trump administration policies. But, according to two sources familiar with the incoming administration’s planning on immigration, Biden has so far not decided whether separated parents will be given the opportunity to come to the U.S. to reunite with their children and pursue claims to asylum.

Herzog, the Department of Homeland Security, and the Justice Department did not immediately respond to requests for comment.