The Boston Police Department reports two detectives possibly stopped a shooting attempt Monday night when they drove by a Geneva Avenue market and noticed a man outside put on a mask, pull out a gun and walk in.

Justin Tompkins, 25 of Dorchester was charged with unlawful possession of a firearm, unlawful possession of ammunition and carrying a loaded firearm for the 9:45 p.m. incident at Ramirez Grocery, 481 Geneva Ave.

Police say Tompkins was wearing an GPS monitoring unit on his ankle, issued out of Dorchester District Court, for an unspecified prior offense.

Police say Tompkins told detectives after his arrest his goal was not robbery but to shoot somebody. Police say his gun had one round in its chamber and four more in a clip. Police describe what happened:

"Detectives notified the dispatcher of their location and as they approached that glass front door the suspect was observed in the front of the store by the checkout counter with a firearm in his right hand. The suspect turned towards the front glass door and one Detective stated, 'Boston Police,' and ordered the suspect to get down on the floor at gun point. The suspect then put the firearm in his right front pocket and then the other detective opened the door and proceeded to cover the detective while the suspect was on the floor so he could be handcuffed.

Police add that a few minutes before the arrest, some men in the store asked a clerk to look outside for somebody matching Tompkins's description.

Tompkins was ordered held in lieu of $100,000 cash bail on the current charges at his arraignment in Dorchester District Court today, the Suffolk County District Attorney's office reports. Even if he should raise that, however, he won't be going anywhere soon - he was ordered held without bail for violating his probation on a 2005 conviction for assault and battery.