Tips from the community helped develop Mercer as a suspect and led police to the home where he was found, said Julie Parker, a Prince George’s County police spokeswoman.
When officers arrived Monday night, Mercer was seen tossing a book bag outside a window, Parker said. Police found the bag, which contained four handguns, authorities said.
Ballistics tests showed that one of the weapons — a Desert Eagle .45-caliber handgun — was used to kill Mercer-Harris and her son, according to police.
Parker said police were investigating a possible motive. She asked anyone with information to call them.
Court records show that Mercer, who has no fixed address, has had extensive contact with police in the past, including convictions in Prince George’s for first-degree assault and carrying a handgun. He was wanted on assault warrants in North Carolina, authorities said.
In April 2005, a Prince George’s Circuit Court jury acquitted Mercer of attempted murder and other offenses in connection with an attack in Forestville. Mercer was accused of driving up to the victim and firing several rounds, wounding the victim.
Mercer-Harris and Harris were found dead early Aug. 4 in the main level of their home in the 11800 block of Hickory Drive. Both had been shot multiple times, police said.
Police were called to the scene by Frank Harris Jr., Mercer-Harris’s husband and Frank L. Harris’s father, about 4:30 a.m., police said. Neighbors said the family lived there for at least 15 years, and a minister at Fort Foote Baptist Church in Fort Washington said the family had attended services each week.
Prince George’s police also announced Tuesday that they have obtained an arrest warrant in connection with an incident Saturday night in which a 23-month-old boy was struck by a stray bullet.
Police were looking for Deven A. Matos, 18, of Fort Washington.
According to charging documents filed in support of his arrest, Matos was one of two men who fired shots about 9:15 p.m. Saturday in an apartment complex in the 3000 block of Brightseat Rd. in Glenarden.
The child, Deondre Johnson, was struck in the face and was in critical but stable condition, authorities said. His mother, Shaketa Johnson, was grazed and not seriously injured, they said.
Staff writer Matt Zapotosky and researcher Magda Jean-Louis contributed to this report.