The embattled chief executive officer of the Capital Area Transit System since late 2009 resigned Monday afternoon.

Brian Marshall provided little explanation for his decision in a one-page letter of resignation to the CATS board, but his departure comes amid recent harsh, public criticism of the bus agency’s performance.

“It is my opinion, that the current environment is not conducive to building a sustainable system,” Marshall wrote. “Therefore, effective immediately, I resign as the CEO of the Capital Area Transit System.”

Marshall, in a phone interview, said he did not want to comment further.

Marshall spent most of the letter defending his record as the bus system’s leader, stressing that CATS is on schedule to deliver the services residents were promised in an April 2012 tax election, including more routes, buses and shorter wait times by 2014.

“Again I will state that my only interest is to build a SUSTAINABLE system that would be delivered in 2014 and the facts again prove that our major endeavors are on target,” Marshall wrote.

He said in the letter that he would remain available to advise management and the board if called upon to do so.

CATS Board President Isaiah Marshall, no relation to Brian Marshall, said the board will meet Thursday to officially accept the resignation.

Asked if the board asked Brian Marshall to resign, Isaiah Marshall would only say, “It was Brian Marshall’s decision to resign.”

Isaiah Marshall added, “We appreciate the work that he’s done up to this point. … This will not impact our ability to move forward.”

Isaiah Marshall said the board on Thursday will vote to give one of its members administrative authority to sign off on contracts while it conducts a quick search of about three to four weeks for an interim CEO. The interim CEO will serve while the board conducts a national search for a permanent CEO, he said.

Brian Marshall pointed to several, noteworthy achievements by his management team during his tenure, including establishment of popular service routes that connected LSU to downtown on LSU football game days. He also said CATS has “gained national attention for our technology innovation” and has secured millions of dollars in competitive grants.

“Through it all, I have been dedicated to building a sustainable system that the city deserves,” Marshall wrote. “Not once was there overspending or deficiencies in our audits.”

Metro Councilwoman C. Denise Marcelle, who has been a vocal supporter of CATS, said losing Brian Marshall will be a detriment to the agency and could slow expansion and improvement plans. She said she suspected Brian Marshall resigned because he grew weary of critics, who she said were out of line.

“I believe they were headed in the right direction,” Marcelle said. “Ultimately it’s going to be a loss for Baton Rouge.”

She said she thinks those who were insisting on a change in leadership at CATS “have another agenda and it has nothing to do with transportation.”

A 20-year veteran of the Chicago Transit Authority, Brian Marshall was hired to lead CATS in 2009. He had spent his last four years in Chicago as manager of the transit authority’s north district.

When he arrived in Baton Rouge, Marshall was immediately confronted with budget shortfalls driven by increasing maintenance and fuel costs and shrinking state and federal allocations.

In 2010, he oversaw the agency in an unsuccessful parishwide tax election to pass a 3.5-mill property tax for CATS that failed 53 to 47 percent.

Last year, with the help of religious and business organizations, and with the blessing of Mayor-President Kip Holden, voters approved a 10.6-mill property tax in the city limits of Baton Rouge and Baker. Zachary voted against the tax; Central and the unincorporated parts of the parish were not included in the tax election.

Transit advocates cheered passage of the tax, hoping the dedicated revenue stream would dramatically improve and expand the bare bones bus service. But in recent months, some of the strongest advocates for the CATS tax have become among the most vocal critics of the bus system’s management.

Together Baton Rouge, a faith-based nonprofit largely credited with helping the passage of the tax, has criticized CATS in recent weeks for missing self-imposed deadlines for service improvements including reduced wait times for riders and new bus shelters.

Together Baton Rouge leader Edgar Cage issued a statement in the wake of Marshall’s resignation, saying: “In times of transition the support of the community becomes even more important. We look forward to working with the CATS board for the goal that brought us together in the first place — building a first-rate transit system for Baton Rouge and Baker.”

Rumors swirled earlier this year about the fate of CATS top management, as the board considered a report from a consulting firm that recommended replacing CATS administrators with contracted employees from a transit management firm.

The consultant said CATS might not be able to compete with the private sector for a top-tier manager and could benefit from a contracted transit firm offering specialized experience.

CATS put out a “request for proposals” earlier this month for a program manager to temporarily oversee the CATS transitions. Isaiah Marshall said Monday the RFP won’t be expanded to provide a larger management role. He said CATS still intends to have a direct hire as a CEO.

In February, the CATS board deferred action on Brian Marshall’s contract when it came up for renewal. He has been working on a month-to-month basis since in lieu of a renewed contract.