Updated: 6:20 p.m. The PMA Group plans to cease its lobbying activities on March 31, according to three sources with knowledge of the firm. The embattled lobby shop, at the center of a federal probe that is believed to involve the relationship between earmarks and campaign contributions, had been among Washington, D.C.’s largest, with lobbying revenues of some $14 million in 2008. The 21-year-old firm, founded and solely owned by former House Appropriations Subcommittee on Defense staffer Paul Magliocchetti, was raided by federal agents in November, and has been hemorrhaging lobbyists since then. Five of its most senior lobbyists, led by Kaylene Green, Dan Cunningham and John Lynch, resigned on Feb. 6 to form their own firm, Flagship Government Relations, and have taken three more PMA lobbyists with them. According to the sources, the firm plans to send out letters to its clients giving them a one-month notice before shutting down. Firm spokesman Patrick Dorton declined to comment Thursday.