The 1,500 immigrant children whom critics claim were lost while in U.S. custody were instead turned over caretakers who didn't respond to the government's follow-up contacts, the Department of Health and Human Services said Monday.

The Office of Refugee Resettlement, which is part of the department, began voluntarily making 30-day follow-up calls in late 2016 to sponsors who had taken the unaccompanied minors, and figures on those who couldn't be reached were disclosed during a Congressional hearing in April. The calls were intended to determine whether the children or their sponsors needed further government services.

"These children are not 'lost,'" Deputy Secretary Eric Hargan said in a statement. "Their sponsors -- who are usually parents or family members and in all cases have been vetted for criminality and ability to provide for them -- simply did not respond or could not be reached."

Many may have ignored the government's efforts because they were in the U.S. illegally and didn't want to risk further contact with federal authorities, he said.

"The U.S. taxpayer can no longer be responsible for being a surrogate parent for every would-be illegal immigrant who crosses our border," the department said in the statement.

"While Health and Human Services houses and cares for unaccompanied children until the point at which unaccompanied children can be released to a sponsor, Congress has not given Health and Human Services the power or the funding to reach far beyond its primary statutory authority," the agency added. "Indeed, that could require billions of additional dollars."

The children cited in the Congressional hearing are not among those separated from their parents because of the Trump administration's efforts to prosecute parents or guardians who entered the U.S. without appropriate papers.

Trump campaigned for president on promises to improve border security and has pledged to build a wall between the U.S. and Mexico.

[Also read: Trump goes off on Democrats for sharing photos of immigrant kids detained under Obama]

