The hunt to reunite families separated under the Trump administration’s “zero tolerance” migration policy is looking for the parents of even more children, an email seen by NBC News says.
The search involves 666 children rather than the 545 known last month, it says.
The policy on illegal immigration at the Mexico border began in 2018 but was pulled months later amid an outcry.
A group of lawyers was appointed last month to find parents, most of whom were deported to Central America.
The coronavirus pandemic has partly hindered the searches.
NBC News said the email, from Steven Herzog, the lawyer heading the mission to reunite families, blames the higher figure on a lack of government information on the additional children whose parents need to be found.
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Mr Herzog says “we would appreciate the government providing any available updated contact information, or other information that may be helpful in establishing contact for all 666”.
Although the “zero tolerance” policy only ran from April to June 2018, it had been running under a pilot programme since 2017 in the El Paso area. Hundreds of parents and children were separated, most of them under the pilot.
As soon as the policy was announced, pictures and audio emerged of children sleeping in cages and crying for their parents, provoking widespread criticism from within the US and around the globe.
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In June 2018, in response to a suit filed by the American Civil Liberties Union (ACLU), a US judge ordered that migrant children and their parents be reunited within 30 days.
But those separated by the pilot programme were not covered by this court order, and their reunification was only ordered last year.
The remaining children have now been released from federal institutions into the care of sponsors - usually relatives or family friends in the US. Dozens of the children are under the age of five.
A House Judiciary Committee report said the Trump policy was implemented with the “full knowledge that hundreds of children would likely be lost to their families forever”.
At the time, President Trump initially defended the practice, saying he “didn’t want” to take children from their parents, but “when you prosecute the parents for coming in illegally - which should happen - you have to take the children away”.
President-elect Joe Biden says he will set up a task force to reunite all migrant families separated under the policy but has not said whether parents will be able to travel to the US to apply for asylum with the children.