PHILADELPHIA — In late November, Miles Pfeffer posted a warning on his Instagram page.

“Work smart not hard make stupid decisions face stupid consequences,” Pfeffer wrote, beneath the photo of a male, his face obscured by fanned-out $20 bills, with what appeared to be a Ruger pistol tucked in his waistband.

According to Philadelphia police, Pfeffer failed to follow his own advice.

Early Sunday morning, federal and local law enforcement officials descended on the home Pfeffer’s mother in Buckingham Township in Bucks County, Pennsylvania, and charged the 18-year-old with murdering Temple University Police Officer Christopher Fitzgerald the night before.

A police affidavit contends that Pfeffer shot the 31-year-old officer near 17th Street and Montgomery Avenue just after 7:12 p.m. Saturday. After Fitzgerald fell to the ground, the suspect stood over him and fired several shots into his face and head — then tried to steal his gun and went through his pockets. What transpired was captured on video.

According to the Philadelphia district attorney’s office, Fitzgerald had chased after Pfeffer while investigating a possible crime near the edge of Temple’s campus. It was unclear what led to that initial encounter. Shortly after Pfeffer shot Fitzgerald, police said, he stole a man’s Infiniti Q60 at gunpoint.

“Give me the keys or I’ll kill you,” Pfeffer allegedly says, according to the police affidavit of probable cause, based on nearby surveillance audio and video recordings.

Two law enforcement sources familiar with the investigation said Pfeffer was later picked up by his mother on Ridge Avenue and driven back to Bucks County. Pfeffer’s brother was with him Saturday night and helped police identify Pfeffer as the gunman, according to the sources, who spoke on the condition of anonymity because they were not authorized to discuss the case.

Pfeffer is facing charges that include murder, murder of a law enforcement official, robbery and carjacking. He is not eligible for bail. The suspect was arrested using Fitzgerald’s handcuffs, a law enforcement tradition for fallen officers.

Fitzgerald, a father of four and avid runner known by friends as “Fitz,” was pronounced dead at 7:27 p.m. He was the first Temple University police officer to be killed in the line of duty.

“Officer Fitzgerald gave his life to selflessly serve and defend this community,” Jennifer Griffin, Temple’s vice president for public safety, wrote Sunday in an email to the university community. “The courage and bravery he displayed highlights the day-to-day sacrifice made by our Temple University Police Officers to keep our community safe. This loss leaves an enormous hole in all of our hearts.”

On Sunday afternoon, Fitzgerald’s relatives gathered around a makeshift memorial site at Bouvier Street and Montgomery Avenue. Down the block from where Fitzgerald was killed less than 24 hours before, his cousin Juan Marrero Jr. reflected on the life of the man he saw as a big brother.

“He wore that badge like he was a superhero, he protected everybody here,” said Marrero, 23. “That’s what he loved to do. So he died doing what he loved to do.”

Marrero said that Fitzgerald’s parents both work in law enforcement. Christopher’s father, Joel Fitzgerald, spent 17 years with the Philadelphia Police Department before moving on to other high-profile positions, including in Allentown, where Christopher had worked as a corrections officer when he was 19. But after a professional stint in Texas, the younger Fitzgerald returned to the city where he was born and grew up.

“He came back down here to the city, Philly. This is everything he cared about,” Marrero said. “Law enforcement was in his blood.”

The Temple community was reeling Sunday, and officials said they have prepared grief counselors and employee-assistance resources for when classes resume on Monday. They anticipate high need.

At the memorial near where Fitzgerald was shot, people placed photos, candles and flowers throughout the day Sunday. A Temple University dispatcher wrapped her arm around a Philadelphia police officer to comfort her. A handwritten note below two roses read: “Thank you for your service, selflessness and bravery. Rest in peace. … You will always be remembered. A true hero.”

District Attorney Larry Krasner said: “Officer Fitzgerald’s life was ripped from him and his loved ones violently and senselessly. As the DA’s Homicide Unit begins the work of holding Miles Pfeffer accountable for his alleged crimes, our Victims Services team will also be extending loving support to Officer Fitzgerald’s family and loved ones.”

Krasner commended Temple University Police, the U.S. Marshals, Philadelphia Police and Bucks County law enforcement officials for quickly apprehending Pfeffer.

“Once again, I call on state and federal lawmakers to apply that same urgency toward addressing the obscene supply and availability of firearms in our communities, which robs people of their lives, futures, and freedoms in a way that no other wealthy peacetime nation tolerates,” Krasner said. “Students, staff, and public safety officers from Pennsylvania to Michigan to California deserve to live freely – absent the danger and trauma that daily gun violence imposes on every American.”

Marrero, Fitzgerald’s cousin, was struggling to understand Sunday why the tragedy had occurred.

Standing with Marrero, Shajia Johnson, 25, another cousin of Fitzgerald’s, said that she was heartened to the see the outpouring of grief and support for Fitzgerald’s family.

“It represents the impact that he had,” said Johnson, who has worked at Temple, as well. “All our colleagues loved him. No one had anything bad to say about Christopher. He was loved, and it shows.”