This afternoon around 2:10PM, officers from District C-11 (Dorchester) arrested suspect, Dereck Gray, 28, of Mattapan on an MBTA bus and charged him with Unlawful Possession of Firearm, Unlawful Possession of Ammunition and Carrying a Loaded Firearm.The arrest was initiated when officers from District C-11 received information from a credible and reliable source that a black male wearing a red sox baseball, and a blue hooded sweatshirt that had just boarded a bus at Ashmont station towards Talbot Ave. was armed with a semi-automatic handgun. While on Talbot Avenue, officers observed the described bus heading in their direction and directed the driver to stop the bus. Officers boarded the bus and therein saw an individual in the back of the bus matching the suspect description. Officers approached the suspect and as they did so, the suspect stood up, reached in his waistband and discarded a black object on the floor. Officers quickly detained the suspect and recovered a semi-automatic loaded firearm right where the suspect had been dropping the item he discarded from his waistband. The suspect will be arraigned at Dorchester District Court tomorrow, Friday, June 11, 2009.