ASTORIA —Firefighters on Friday mourned the alleged murder of an EMS lieutenant who died after suffering more than 20 stab wounds inflicted during an attack Thursday.

EMS Lt. Alison Russo-Elling, 61, served with the FDNY for 25 years, and was among first responders to the 9/11 terror attacks.

She died at Mount Sinai Queens Hospital. The random attack happened at 2:15 p.m. Thursday at the corner of 20th Avenue and 41st Street, near Russo-Elling’s assigned EMS Station, No. 49.

The suspect, Peter Zisopoulos, 34, was charged with murder on Friday after two witnesses told police shortly after the attack that he is a local resident. Police arrested him at his nearby apartment.

Acting Fire Commissioner Laura Kavanagh called the attack “barbaric and completely unprovoked.”

“Members of EMS serve only to help and save other people’s lives,” Kavanagh told reporters at the hospital. “To be attacked and killed in the course of helping others is both heartbreaking and enraging for our department in ways I cannot describe.”

Police Commissioner Keechant Sewell, also at the hospital, said details were “preliminary” and “subject to change.”

While police did not immediately release a motive, some media outlets quoted sources who claimed Zisopoulos was schizophrenic.

A surveillance camera video of the attack shows a large man approaching Russo-Elling, who was in uniform during a lunch break near the station. The man, believed to be Zisopoulos, knocked her to the ground.

He then pounded a blade at least 20 times to her upper torso, stood up, and then briefly wandered in and out of the camera’s view. He then walked from the scene.

Chief of Detectives James Essig said two people “who know the perpetrator” witnessed the attack. One followed him to a nearby apartment on 41st Street and told police.

“Members of the hostage negotiating team and Emergency Service Unit were able to talk this person out of the apartment,” Essig said. “He was taken into custody and is presently in the 114th Precinct where the investigation is ongoing.”

Mayor Eric Adams opened the press conference with Kavanagh and Sewell at the hospital.

“We lost one of our heroes,” he said. “Our hearts go out to the family, her colleagues, and the City of New York.”

Russo-Elling reportedly was about to retire. She is the third FDNY member to die this year while on duty. Firefighter Jesse Gerhard collapsed at his firehouse while on duty in Far Rockaway on Feb. 16. Firefighter Timothy Klein died on April 24. He suffered fatal injuries while battling a house fire in Canarsie.

The EMS lieutenant was a resident of Long Island and a mother; survivors include her parents, according to an FDNY news release.

“Lt. Russo-Elling exemplified FDNY EMS,” Kavanagh said at the hospital. “She was a World Trade Center first responder. She was cited multiple times for her bravery and her life-saving work, and she was absolutely beloved on this job.

“She is the 1,158th member of the FDNY to die in the line of duty and the second member of EMS to die in the line of duty in the last five years. Our hearts break for our entire department and her family.”

Kavanagh thanked hospital personnel “who did absolutely everything in their power” in their efforts to help Russo-Elling. She thanked the NYPD for the quick arrest of the suspect.

Sewell added, “This deadly, senseless, broad daylight attack on a uniformed EMT member is a direct assault on our society. It is the latest consequence of the violence that we relentlessly fight in our city.”

“We can never tolerate this violence in our city,” she concluded. “It has to and will be stopped.”

Bishop Robert Brennan on Friday issued a statement about Russo-Elling.

“I join with the men and women of the @FDNY in mourning one of their own,” he stated on Twitter. “I am horrified at this brutal murder of one of New York’s Bravest. We are grateful to God for the way she brought His love and care to people.”

Msgr. Jamie Gigantiello, a chaplain for the FDNY, went to the hospital to help Russo’s family and colleagues.

“We believe that it was totally random,” Msgr. Gigantiello said. “She went into this deli to purchase a sandwich. Supposedly the person who did this to her was also in there. And he came out and just started to stab her.”

The chaplain said Russo-Elling’s parents and daughter arrived later from Long Island. They had private time before her FDNY colleagues were allowed to see her.

“They entered the room and they saw her there, and it became real,” he said. “You see people just breaking down — grown men crying.”

Msgr. Gigantiello said he counseled Russo’s colleagues from Station 49.

“They just saw her moments before and then they got the call she was stabbed,” he said. “You’re in total shock first of all, then you begin to think how senseless this is. 

“Because they weren’t putting out a fire or rescuing someone and bringing someone to the hospital. But something like this is a random act of violence, it’s just so hard to swallow. And you say why? How are these things happening?

“The first words from the father when he came in, was ‘I don’t understand. She gave her whole life helping people. And this is what happens to her?’”

Msgr. Gigantiello said violence on New York City streets is escalating, and much of it is random and inflicted by mentally ill people. He said the city must find ways to balance compassion for the sick, while protecting everyone.

“Obviously, the person who did this to Alison was mentally disturbed,” he said. “And mental illness is a big issue. We have to take everything into account, but also we have to keep our streets safe.”

The chaplain said Russo was Christian, not Catholic. He added that the FDNY family assistance unit was helping her family with funeral arrangements. He said the funeral would be a major line-of-duty funeral, similar to the services for Klein earlier this year.