Family members confirmed the girl's identity as Monae Turnage, according to local newscasts. They had reported her missing early Sunday.
The body was found about 6 p.m. in the backyard of a two-story, brick rowhouse in the 1600 block of Cliftview Ave., said Detective Donny Moses. Monae's aunt, Paulette Marshall, confirmed on newscasts that the girl's 16-year-old brother and neighborhood youths found the clothed body while looking through alleys.
Moses said the body, covered with plastic trash bags, had suffered from trauma, but he was not specific. He confirmed the East Baltimore girl's family had reported her missing early Sunday.
"She left home last night to go rollerskating and never returned," Marshall told WJZ.
Marshall said the family became alarmed and called police at 1 a.m. to report Monae missing "because she didn't come home and [she's never done] anything like this before and we knew that."
Police are interested in questioning the people with whom the girl was supposed to have gone skating, Moses said, "in hopes of finding more information about how she died."
Moses said the girl was declared dead at the scene. The state medical examiner's office is to conduct an autopsy Monday.
On Sunday night, neighbors stood on porches or in yards adjoining the alley. Several said they did not see or hear anything unusual Sunday and didn't know about a body being found until they saw police cars in the area and officers knocking on doors.
Shawnda Robinson said Monae was a student at William C. March Middle School, several blocks from where the body was found.
Michelle Davis, who lives several houses away from the body was found, said "I feel so sad because I have a 13-year-old daughter and a 15-year-old daughter and that's too close to home. That's a baby."
Neighbors pointed to the last time a child was fatally injured in the Darley Park area, in May, when 12-year-old Sean Johnson was killed by a stray bullet while watching television with friends on the front porch of a house in the 1700 block of Cliftview Ave. Police said he was caught in the crossfire of two men shooting at each other.
Neighbor John Lopez told WJZ he thought the girl's killer was "a monster."
"What kind of human being would do something like that to a 13-year-old child?" he said.
Police ask anyone with information to call Baltimore City homicide detectives.
WJZ-TV contributed to this article.