MARTA’s expansion plans for Atlanta and Clayton County are substantially short of revenue, making it a “monumental challenge” to deliver the promised projects in a timely manner, according to a former agency official.

In a social media post Friday, former Deputy General Manager Josh Rowan said a recent independent financial analysis concluded MARTA’s Atlanta expansion program faces a revenue shortfall of more than $1 billion. He said its Clayton County program faces a shortfall of more than $160 million.

Rowan cited “a third-party financial capacity analysis draft report” dated last month.

On Saturday, MARTA confirmed it has hired a third-party vendor to prepare a financial model for the agency’s future capital needs, including the Atlanta and Clayton expansion programs and maintaining the existing system. In a statement to The Atlanta Journal-Constitution, the agency said the model will “allow MARTA to better plan for the timing, cost, and scope of future programs by modifying certain assumptions and inputs based on real-world conditions.”

“The model is not complete, and the numbers referenced by Mr. Rowan are one outcome that could be created using a certain set of assumptions and inputs as showcased by the vendor during a walk-through of an incomplete model,” MARTA said.

Rowan provided his own statement Saturday, saying he looked forward to General Manager Collie Greenwood’s “presentation of this now-completed model” during MARTA’s quarterly briefing to the Atlanta City Council in March.

Until last week, Rowan oversaw MARTA’s expansion plans in Atlanta and Clayton County. His comments came in a LinkedIn post in which he said he was fired without cause on Jan. 5.

In the post, Rowan thanked his MARTA colleagues and said they faced “a monumental challenge to deliver these voter-approved portfolio of [transit expansion] projects in any sort of timely manner.”

Though it’s difficult to assess the impact of any shortfalls, Rowan’s comments are the latest evidence that MARTA is struggling to live up to the expansion plans it has unveiled in recent years.

Atlanta voters approved a half-penny sales tax for a 40-year transit expansion in 2016. Two years later, MARTA unveiled a list of projects that included 29 miles of light rail, 13 miles of bus rapid transit, station renovations and other improvements.

But over the last year MARTA has been backing away from its light rail plans. It chose rapid buses instead of light rail for a Campbellton Road line, and it’s considering rapid buses as well as light rail for its proposed Clifton Corridor line. The agency is moving ahead with plans for an Atlanta streetcar extension to Ponce City Market.

Inflation and unexpected costs have increased the price tag for MARTA’s Summerhill bus rapid transit line. And last May the agency disclosed that it has spent more than half the proceeds of the new Atlanta sales tax on enhanced bus service and other operations, a move that could limit the funds available for building new transit lines.

In recent months MARTA and Atlanta officials have been renegotiating the expansion project list in light of new financial realities. It’s unclear when those negotiations will be finished.

Clayton County voters agreed to join MARTA in 2014, and the agency had planned to build a commuter rail line from East Point to Jonesboro and Lovejoy. Those plans fell apart after the Norfolk Southern railroad said it would not allow the passenger rail line to use its track.

With the blessing of Clayton officials, MARTA recently chose rapid buses as its preferred alternative for the transit line.