BOSTON MAYOR Michelle Wu took her bid for three fare-free bus routes to Washington on Tuesday, briefing US Transportation Secretary Pete Buttigieg on a snag caused by guidelines issued by the Federal Transit Administration.

Wu, who was attending a White House function on Tuesday, tweeted a link to a CommonWealth story on the fare-free bus controversy and added: “Guess who brought this up today at the White House?”

It’s unclear whether Buttigieg promised any action on the matter because a spokesperson for the Federal Transit Administration on Wednesday issued a statement basically confirming the MBTA’s understanding of existing guidelines. As for Wu’s tweet, the spokesperson suggested contacting the mayor’s press office.

The city is currently financing an MBTA pilot project offering fare free service on the Route 28 bus and the City Council recently voted to spend an additional $8 million in federal funds to eliminate fares on two more T bus routes and extend the service on all three routes for two years.

The plan hit a snag when the MBTA raised concerns about a Federal Transit  Administration guideline requiring pilot projects to be made permanent or canceled after six months. The T is concerned that the proposed Boston fare-free pilots, if they run beyond six months, would become permanent and could trigger a negative equity finding if fares rise again when the federal funds run out.

The Federal Transit Administration spokesperson said agency guidelines require recipients of federal funding to conduct a fare equity analysis for all permanent fare changes. She said any fare pilot that goes longer than six months is considered to be permanent and therefore would require a fare equity analysis. If the analysis reveals a disparate impact on the basis of race, color, or national origin, the MBTA would be required to correct it, the spokesperson said.

“The guidelines … would not prevent MBTA from raising the fare back to the regular fare at the end of a two-year pilot,” the spokesperson said. “Fare changes are common throughout the transit industry, including after pilot fare reductions. At the conclusion of the two-year pilot, if MBTA needs to reinstate a fare on these routes, it would need to conduct a fare equity analysis again.”

T officials have raised concerns that this second fare equity analysis would find disparate impact if the funding for the pilot runs out and the T is forced to raise fares back to their previous level.