Tesla and SpaceX CEO Elon Musk said Thursday that The Boring Company, a side project of his focused on tunneling infrastructure, received “verbal government approval” to construct an underground Hyperloop route from New York City to Washington, D.C. The trip would take just 29 minutes, Musk said, with “up to a dozen or more entry/exit elevators in each city.” The Hyperloop is a yet-unproven high speed train concept that would theoretically be able to travel over 700 miles per hour.

Musk said the route would serve four cities: New York, Washington, Philadelphia and Baltimore. But when contacted by Fortune, representatives for these cities said they have not approved such a project. Moreover, they also said that they have not yet held substantive discussions on the proposal.

“This is the first we’ve heard of it, but we can’t wait to hear more,” LaToya Foster, press secretary for the mayor of Washington, D.C., told Fortune.

“Elon Musk has had no contact with Philadelphia officials on this matter. We do not know what he means when he says he received ‘verbal government approval,'” said Mike Dunn, deputy communications director at the City of Philadelphia.

“Right now, I would characterize any conversations as preliminary and not extensive,” said Anthony McCarthy, director of communications for the Baltimore mayor’s office. He added that the City of Baltimore has not given Musk or The Boring Company approval for the project.

“This is news to City Hall,” said New York City Press Secretary Eric Phillips in a tweet about Musk’s announcement. He later added: “The entirety of what we know about this proposal is what’s in Mr. Musk’s Tweet. That is not how we evaluate projects of any scale.” Phillips’ deputy added that Musk “must be joking.”

The White House did not confirm Musk’s tweet either. “We have had promising conversations to date, are committed to transformative infrastructure projects and believe our greatest solutions have often come from the ingenuity and drive of the private sector,” the White House said in a statement to The New York Times.

Shortly after his original message — and facing a flurry of questions on the matter — Musk walked back his announcement. “Still a lot of work needed to receive formal approval, but am optimistic that will occur rapidly,” he tweeted about an hour and a half after his original post.

Musk later appeared to be drumming up support for the measure, tweeting that “if you want this to happen fast, please let your local & federal elected representatives know. Makes a big difference if they hear from you.”

Still a lot of work needed to receive formal approval, but am optimistic that will occur rapidly

— Elon Musk (@elonmusk) July 20, 2017

From where Musk may have received his “verbal govt approval” remains unknown. A Boring Company spokesperson told Fortune that the project has received support from “key government decision-makers,” but did not provide further specifics.

If Musk could indeed build a high-speed route between the United States’ economic and political centers, it could be a game-changer for the east coast. But he would first need approval from a great many number of agencies and other stakeholders, as urbanist and Massachusetts Institute of Technology doctoral student Yonah Freemark pointed out. “Approval needed from: Federal [Department of Transportation]; 6 states; 17 counties; numerous cities; hundreds of elected officials,” said Freemark on Twitter. “Definitely happening rapidly.” Musk has demonstrated a unique knack for overcoming expectations. But even he may not be able to beat city hall.