The rider, 33-year-old Keylin Hall of Dorchester, now faces his fourth firearm charge.

Officers were helping disperse a large crowd near 996 Blue Hill Avenue around 4:04 a.m. Saturday, July 11, when they clocked a moped rolling south toward Mattapan Square with its lights out. As they moved in and immobilized the bike, Hall allegedly abandoned it and took off on foot down the sidewalk toward Callendar Street, one hand pinned to the right side of his waistband the entire way, according to police.

He didn't get far. Officers caught up and detained him, and the frisk that followed allegedly turned up exactly what his hand had been guarding — a Glock 44 .22 caliber pistol wedged inside his pant leg, loaded with ten rounds of live ammunition.

Then came the moped. As officers moved to tow it, they say they discovered five small plastic bags of what they believe is crack cocaine and two more containing suspected fentanyl.

Hall is expected to be arraigned in Dorchester District Court on charges of:

He was also cited under Massachusetts's uniform auto laws for the moped violations — the missing helmet, dark lights, and lack of registration that started the entire encounter.