Police were still hunting for the second suspect Friday night. He has been identified as Nalik Shariff Scott, 30, of the 1300 block of South 24th Street in South Philadelphia.
It was about 8 p.m., just minutes before closing time, when two gunmen burst into the family-owned store at 50th and Parrish Streets, police said.
Porfirio Nunez, the owner, who had turned 50 that day, was shot first in one of the aisles, Clark said. Scott fired the first bullet into him, Clark said.
Muhammed executed Carmen Nunez, 44, and her sister Lina Sanchez, 48, as they tried to crawl for cover, police said, then fired the fatal bullet into Nunez as he stepped over the dying man.
The gunmen did not shoot Nunez's two teenage daughters, who had ducked behind the counter. The killers ran from the store to a car around the corner. They got no money.
"We had a family brutally executed for nothing," Clark said. "We are very, very happy to have made this arrest."
Clark described the arrest as a collaborative effort of police units.
In January, Clark organized a task force of detectives to take a fresh look at the investigation, then four months old.
Last week, investigators got their strongest break when they linked the slayings to an August stickup of a Southwest Philadelphia corner store and the shooting of a North Philadelphia store clerk earlier in the summer.
Surveillance footage of the Southwest robbery, at the Jaquez Grocery at 62d and Reedland Streets, provided investigators with clearer images of the assailants' faces.
Thursday, more than 100 narcotics officers conducted drug sweeps through Southwest and West Philadelphia, aiming for intelligence on recent shootings and homicides, including the grocery-store killings, said Deputy Commissioner William Blackburn.
Around 3:30 p.m., police raided a home on the 6200 block of Reedland, where Muhammed lived with his girlfriend just down the block from the Jaquez Grocery. They had received intelligence that drugs were being sold from the residence. Muhammed was arrested inside with a small amount of marijuana, investigators said.
He told officers that he didn't want to go back to jail and that he had information on shootings, police said.
The narcotics officers took him to Southwest Detectives, where investigators strongly suspected he was one of the robbers in the Jaquez surveillance footage. He was then brought to Police Headquarters, where homicide detectives grilled him about the triple slaying.
He was arrested again within an hour, Clark said.
Clark did not say whether Muhammed confessed, but said police had "more than enough" to charge him in the slayings.
Contact staff writer Mike Newall at 215-854-2759, email@example.com, or @MikeNewall on Twitter.
Inquirer staff writer Allison Steele contributed to this article.