The Department of Homeland Security has only found $20 million in existing funds to put toward construction of a wall on the United States’ border with Mexico, according to a document obtained by Reuters.

The news outlet had previously reported that the wall would cost $21.6 billion.

The agency's document said that the $20 million would be enough for prototype contracts, but not to begin building the structure.

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The DHS limited is search to extra funds within its own $376 million budget for border security fencing, infrastructure and technology, Reuters reported, so that it wouldn't need to ask Congress to repurpose other funding.

The document was prepared by DHS and distributed to congressional budget staff, according to the report.

Trump last month signed an executive order calling for construction of the wall, a signature promise of his presidential campaign, which would be financed using “existing funds and resources.”

The president insisted throughout the campaign that Mexico would pay for the wall, though he later claimed that Mexico instead would reimburse the United States in the future.