The executive director of the $1.6 billion MBTA Retirement Fund, which has come under increased public scrutiny in recent months, is stepping down after 10 years on the job.

Michael Mulhern submitted his letter of resignation Monday to the board that oversees the privately managed fund for current and past MBTA workers. In it, Mulhern said he planned to leave the post on Aug. 5 and pursue other opportunities in the transportation industry.

He wrote that over the last year the fund's staff has "patiently and professionally endured unprecedented internal scrutiny and ill-informed public criticism."

The fund has come under fire for a lack of transparency. A whistleblower report last year questioned whether it had overstated its financial health.

The fund later released a consultant's report rebutting the whistleblower's findings.