It was just supposed to be a robbery.

That was the first line of the confession Ibrahim Muhammed signed in the wee hours of the morning on Feb. 10, 2012, Police Detective Thomas Gaul told a Philadelphia Common Pleas Court jury Monday morning.

But what was supposed to be "just a robbery" - at Lorena Grocery in West Philadelphia in September 2011 - left three people dead. And in a homicide interrogation room five months after the slayings took place, prosecutors say, Muhammed admitted to it all.

Defense attorneys for Muhammed and his codefendant, Nalik Scott, say that confession was false - taken while their client was off his medication for schizophrenia. Both men are innocent, they say, the victims of mistaken identity.

In cross-examination, they noted that Gaul lied to Muhammed at the beginning of his interrogation, and suggested that homicide detectives had not followed up on other promising leads once Muhammed confessed.

Gaul, continuing his testimony from Friday, read the confession on the stand Monday.

Muhammed told him that he and Scott had targeted the bodega at 50th and Parrish Streets because they believed the owners were selling "weight," or drugs, out of the shop - which wasn't true, Gaul said.

But, Muhammed told him, he and Scott expected a big haul, Gaul said. So, he said, they went to Lorena just before 8 p.m. on the night of Sept. 6, 2011, where Porfirio Nunez, 50; his wife, Juana, 44; and his sister Lina Sanchez, 48, were working, Gaul testified.

The couple's daughters, Jessica and Laura, were at the cash register when Muhammed and Scott came in, the daughters testified last week.

Scott confronted the girls, while "I held the women hostage to make sure they didn't have guns or knives," Gaul said Muhammed told him. Then, Muhammed said, he heard a gunshot from the front of the store, Gaul testified.

"I didn't know where they were coming from, or if they had muscle," Gaul read from the confession.

Muhammed said he opened fire, Gaul said.

"I was not aiming anywhere. Just squeezing," he read from the confession.

Muhammed said that later, Scott told him he had fired the first shot at Porfirio Nunez because the bodega owner, running toward his daughters' cries for help, had tried to pull a gun, Gaul testified.

Fleeing the store, Muhammed said, he accidentally shot Nunez while trying to pull his own gun - which was burning hot - away from his body, Gaul testified.

Muhammed told him that the two men ran from the store and drove back to his neighborhood, where he bought two cigarettes from a store and went home, Gaul testified.

"Everything happened so fast in that store," Gaul read from the confession.

In cross-examination, Muhammed's defense attorney, Anthony Voci, pressed Gaul about the fact that he lied to Muhammed before his confession. Detectives had connected the Lorena killings to two other robberies, Gaul said, and he told Muhammed that police had found his fingerprint at one of the robbery sites. They hadn't.

Gaul said he also suggested to Muhammed that police had obtained video of the bodega killings.

"So that was an intentional falsehood you put into his head?" Voci asked.

"That's correct," Gaul said. He had been trying, he testified Friday, to see how much Muhammed knew about the case.

Voci asked Gaul whether he had asked Muhammed about his psychiatric history - and whether he had dealt with suspects with mental-health issues before.

"I could tell if someone did have mental-health issues," Gaul said. "And [Muhammed] was very clear and concise. Almost like a businessman."

Defense attorneys suggested that homicide detectives had not followed up on other leads during the case.

Although Muhammed admitted in his confession to the two other robberies connected to the Lorena killings, Gaul said, he said he had committed them with a man named Tyrone instead of Scott.

"Did you ask him, who is Tyrone? Try to get his cellphone number?" defense attorney Jack McMahon asked.

"No," Gaul answered.

McMahon questioned Gaul about a video taken from a bodega where Jessica Nunez worked in the months after the murders. Nunez took several jobs at area bodegas after her family was killed, hoping to spot the gunmen. A few days before Muhammed's confession, she fainted in a bodega at 72nd and Haverford Streets after serving a customer.

Gaul said Nunez told him the customer looked extraordinarily like the man who had shot her father, and believed he could be a relative of the gunman.

But in the Police Department's written chronology of the case, a detective wrote that Nunez had "sighted the male" - suggesting that she had seen her family's killer, McMahon said.

The defense has argued that the man Jessica Nunez saw could be the real killer, and that police did not follow up on the tip after Muhammed confessed.

Gaul said that if Nunez had told him she saw the man who killed her family - not just someone who looked like him - he would have sent SWAT teams to the neighborhood.

"It's poorly written," he said of the department's paperwork on the incident, which he said was written by another detective. "And that's my responsibility. It should never have said, 'That was the guy.' "

The prosecution is expected to rest its case Tuesday morning.

215-854-2961 @aubreyjwhelan