A new Netflix documentary tells the story of a serial-killer nurse. The colleague who brought him down shares what it was like.

• The nurse Charles Cullen admitted to killing dozens of patients. Cops say he may have more victims.
• Another nurse, who worked in the same ICU, helped detectives end his 16-year killing spree.

In 2002, Charles Cullen, a nurse at Somerset Medical Center in New Jersey, was one of three employees who were chosen to appear in a marketing campaign for the hospital.

Cullen's face was front and center on the pamphlet. Amy Loughren, Cullen's closest colleague in the ICU, said that he enjoyed the attention. "I used to call him our Somerset spokesmodel," the former nurse told Insider. "He was very proud of the fact that they'd put him in their advertising brochure."

Cullen was the chief suspect in a series of suspicious deaths in hospitals in New Jersey and Pennsylvania. He'd been caring for the patients — who ranged in age from their early 20s to their late 90s — when they died.

Loughren, a mom-of-two who now runs a spiritual healing company, is featured in the Netflix documentary, "Capturing the Killer Nurse," released this month. The film tells the story of the hospital serial killer and asks how he got away with murdering dozens — if not hundreds — of patients over a period of 16 years.

Loughren and Cullen became friendly when they worked in an ICU

The film's director, Tim Travis Hawkins, told Insider that the killer — whom he described as "ordinary and quiet" — craved notoriety. He said that Cullen thought his current life was "boring."

"He comes across to me as quite an infantile man who is entitled to an insulting degree," Hawkins said. "His ultimate act of cowardice was refusing to give any sort of acknowledgment to the victims' families when they confronted him in court."

Loughren first met Cullen in 2002 when he got a job in the critical-care unit at Somerset. "He had a very thoughtful demeanor and spoke very softly and never, ever raised his voice," she said.

Loughren, who supervised Cullen and other nurses on the team, told Insider that the friends had each others' backs.

One time, she said, an older patient — "a really beautiful woman who had come in because her pacemaker had malfunctioned" — was admitted to the ICU. Nurses had noted on her chart that she was allergic to lidocaine.

Loughren walked into her room a few days later to find Cullen by her bed, holding a syringe. He'd injected the woman with the painkiller.

"I saw that she was arresting on the monitor and yelled 'Code Blue' down the hallway," Loughren said.

The staff tried to save her life with CPR, but she was pronounced dead. The attending physician, Loughren said, accused the team of carrying out a "poor code." She said the doctor shouted, "Who the hell gave her lidocaine?"

Loughren said she never became suspicious that something was wrong

Cullen thanked Loughren for keeping him in a job. She said that she thought that it was a genuine mix-up. Cullen worked at Somerset for a total nine months.

"I knew that there were a lot more codes in that particular ICU versus other ICUs that I'd worked in," Loughren told Insider. "But I wasn't suspicious. I thought, 'These people are sick — they just took a turn for the worse.'"

In July 2003, the New Jersey Poison Information and Education System found at least four suspicious overdoses at Somerset. It warned hospital management that an employee could be killing patients.

The hospital had already begun to investigate discrepancies in its dispensing system for drugs. Computer records showed that Cullen had long been requesting medication that had not been prescribed. But they kept him on for a three more months. He was fired in October 2003.

Loughren said people were confused by his dismissal. "We had no idea what happened," she said. She only realized that something was wrong when the staff at the ICU was questioned by detectives.

"At first, I thought that they were trying to pin something on Charlie, like narcotics theft," Loughren said. "It didn't occur to me that there was something more sinister going on."

But when the police produced the evidence — including Cullen's unexplained requests for certain medications — she knew "something really, really bad was happening." Loughren said the detectives told her they suspected that Cullen had killed many patients, "not only at Somerset, but four other hospitals."

Loughren said Cullen looked smug when she challenged him about the murders

Loughren told Insider that she felt shocked and in disbelief. "I was in denial that my reality was so skewed, I was close friends with a monster," she said.

Loughren agreed to wear a wire to try and get Cullen to confess to the crimes. She met him at a restaurant.

At this point, Loughren said, journalists knew that Cullen was suspected of "hurting people at the hospital." She sat at the table with her former colleague as he proudly pointed to the reports in the papers.

"I said, 'I'm not stupid. I know that you're guilty,'" Loughren said. "Let's go to the police station together. I'll stay right with you. I care so deeply about you."

She said that Cullen "became someone else" in that moment. "He was smug, but there was emptiness there that I had never seen before," she said.

He was arrested and finally admitted his guilt at the police station. Loughren was left alone with him in an interview room. She told Cullen that she was scared she might be implicated in his crimes because she knew him so well.

Loughren told Insider that a lot of her anger was directed at herself for not protecting the patients.

Loughren criticized hospital chiefs for allowing Cullen to pass from job to job

Detectives found that the lidocaine death was one of at least 29 murders committed by Cullen. He has confessed to 20 more in prison. The death count, according to "Capturing the Killer Nurse," could be in the hundreds.

"Nobody will ever know for sure," the director Hawkins said. Pathologists discovered that Cullen had killed his victims mostly with insulin, the heart drug digoxin, and the allergy drug epinephrine. The latter can cause an irregular heartbeat and a buildup of fluid in the lungs.

The Cullen murders caused outrage when it became public that the killer had struck in five different hospitals where he'd worked. Investigators found that some of the institutions had dismissed accusations by his coworkers and bereaved families that Cullen was behind certain deaths. One hospital fired him but agreed to give him a "neutral" reference as part of the agreement.

Loughren said she supports the judge's decision to give Cullen a total of 18 consecutive life sentences. "I know that my friend Charlie is in the place that he needs to be," she said.

As for her role in bringing him to justice, she said, "I know that it was the right thing to do."