By Wednesday night last week, the suspects in the killing of 2-year-old Nikolette Rivera were sequestered in separate interrogation rooms at Police Headquarters. And the case that had gripped a city was coming together.

Homicide detectives from 1 Squad had the leverage they needed. They had video footage of a silver car speeding away from the block where someone had fired an AK-47 into the child’s house, hitting her in the head as her mother held her, and wounding her mother and a cleaning man. And for once, the car was distinctive, with damage on the right side, a buckled trunk held together by red tape. They easily traced it. Even without that, they had a name, or at least a nickname. The man in the car was Curly Top, tipsters had told them.

And the detectives believed Curly Top was sitting in one of those interrogation rooms. His real name was Freddie Perez. The alleged shooter, his accomplice, was in the other one: Tayvon Thomas. Their confessions came quickly. They said they shot up the Riveras’ Kensington house over a drug dispute. They were aiming for Nikolette’s father.

In their statements, both men, independently of each other, offered investigators a startling admission. The AK-47 that Thomas had aimed at the Riveras was supplied by a man named Francisco Ortiz. Investigators quickly realized he was the prime suspect in the other horrific shooting that had shocked the city that weekend, just a day before Nikolette was killed: an 11-month-old baby shot in Hunting Park and critically injured in another drug beef.

It’s the kind of detail that chills to the bone even in a city that can be tragically inured to deadly violence. Two babies, shot in the head, in completely separate cases, in completely different parts of this city of 1.5 million — yet inextricably linked by the circles through which guns so easily move in Philadelphia.

“It tells us that there’s a lot more to crime than random shootings,” said Caterina Roman, an associate professor of justice at Temple University. “That individuals involved in street crime likely know each other and are connected in some way. That there’s a street scene and a code — people engaged in violence are going to know how to easily get a gun.”

But the detectives in 1 Squad could not afford to dwell on that grim link for long. The connection had little bearing on the larger case. Detectives had not recovered the gun or turned up evidence that Ortiz knew what that AK-47 was going to be used for.

Perez and Thomas had told detectives they were going after Nikolette’s father, Nikolai, a leader of their former drug crew, in an effort to take the corner he sold on — and because they had gone to prison for that crew and weren’t taken care of. No lawyers, no bail money, no money on their books.

So Perez, who had 17 previous arrests and was still on the street, and Thomas got an AK. Ortiz was just someone in Kensington who could provide it. Just another piece of the puzzle in a city where guns recovered in murder investigations sometimes match ballistics from six or seven other cases.

“It’s going to shock the general public,” said Lt. Philip Riehl, the commander of 1 Squad. “But there’s very little that shocks us.”

It wasn’t the gun that shocked Riehl. It was the children who were killed on his shift last week. Little Nikolette; Damaya and Maxilla Alcindor, a baby and a toddler, shot to death by their mother in the Northeast; and Zyqueire Echevarria, a 15-year-old shot on a street corner in South Philly, the type of killing that no longer warrants a headline in this city.

“I have supervised 1,800 murder investigations,” Riehl said. “I have never seen a week like this.”

Shock can feel like it’s in short supply in Philadelphia when it comes to gun violence. We lend our outrage to the horror of one killing — to an eerie coincidence that might startle, or a grim detail that makes its way into the headlines. We do this at the expense of other victims. Because the detectives of 1 Squad are busy. Since last week, since the week that shocked even hardened investigators of the homicide unit, 10 more murder cases have landed on their desks.