Atlanta, Georgia (CNN) -- Bobby Maurice Tillman, the Georgia teen stomped to death outside a home where a party was being held, died from a lacerated heart, Douglas County Coroner Randy Daniel told CNN Tuesday.

He said the 18-year-old likely suffered a break in a rib bone, which in turn punctured his heart. If the young man had not been stomped, he probably would not have been injured in that way and would have survived the beating, Daniel added.

Police say Horace Damon Coleman, 19, Emanuel Benjamin Boykins, 18, Quantez Devonta Mallory, 18, and Tracen Franklin, 19, stomped and beat Tillman outside a party in metro Atlanta over the weekend. The four men are facing felony murder charges.

Douglas County Sheriff Phil Miller has said the attack happened after some girls got into a fight, and one of them hit a boy. That boy, according to Miller, said he wasn't going to "hit a girl," but the next guy who came by was going to get "beat down."

Tillman walked by.

Miller said the victim was a "little guy, 18 years old, 5-foot-6, weighed 125 pounds."

"He was small in stature, but ... his spirit and his strength were bigger to me than this world," Tillman's mother, Monique Rivarde, told CNN Tuesday. "My son was bullied by four cowards."

No drugs or alcohol were found at the scene, according to the sheriff, who said he doesn't expect any more charges in the case.

CNN's Tristan Smith contributed to this report.