An Air Force veteran has racked up millions of more dollars on his fundraising page for U.S. President Donald Trump's U.S.-Mexico border wall, though it's unclear how the U.S. government would get the money.

Brian Kolfage's GoFundMe page has raised $13 million as of Friday afternoon to build the wall. Its goal is $1 billion. Kolfage says on the site that he contacted the Trump administration about how to deliver the funds.

White House officials did not immediately return an email seeking comment Friday.

The Treasury Department says citizens can mail money as "gifts to the United States." But it's not clear whether the Homeland Security Department can accept gifts.

Kolfage hasn't responded to multiple requests for comment.