Congress believes the Pentagon has $21 billion in unobligated military construction funding—money that has been appropriated by the legislature and set aside for specific projects but not yet issued—that the president could use to build the wall over objections from Congress, according to two congressional aides.

The money, which would be taken from construction projects such as hospitals, family housing, and maintenance, would not be replenished until Congress passes another defense appropriations bill, the aides said. This could leave critical infrastructure improvements in limbo…

The $21 billion comes from funds in the Defense Department’s budget that were appropriated for various military construction projects over the last five years but not yet spent, the aides said. The largest chunk, $10 billion, is from the appropriation for fiscal year 2019, which Congress just passed and is largely unspent. The rest, about $11 billion, comes from the previous four years.