When a gunman opened fire inside a Walmart in Thornton Wednesday night, shoppers screamed and ran for cover — and others pulled out their own handguns.

But those who drew weapons during the shootings ultimately delayed the investigation as authorities pored over surveillance videotape trying to identify the assailant who killed three people, police said Thursday.

Joe Amon, The Denver Post The Fobb family Angelique 13, Jason Fobb, Destiney 9, Marlena Fobb. Marlena Fobb is hugged by her daughter Destiney 9, as she tells her story of being in the check-out line with her husband Jason next to a man that was shot and killed in the shooting at Walmart in Thornton. "He saved my life, she said, talking of her husband, If he hadn't of threw me to the ground we were gone."

Provided by Thornton Police Department Scott Ostrem

Provided by Wheat Ridge Police Department Scott Ostrem in 2013



Provided by Wheat Ridge Police Department Scott Ostrem in 2014

John Leyba, The Denver Post Rochelle Ginsner holds her daughter Danielle Carey, 16, an employee, after they were released from the parking lot at Walmart after a shooting at the store on Nov. 1, 2017.

Helen H. Richardson, The Denver Post Walmart employees and customers head away from the scene outside of the Walmart store where a shooting occurred inside the store at 9901 Grant Street on Nov. 1, 2017 in Thornton.



AAron Ontiveroz, The Denver Post Darlene Jackson sits on an overturned shopping cart as she and fellow shoppers and employees gather outside after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton on Nov. 1, 2017.

AAron Ontiveroz, The Denver Post Employees Jose Delgado and Alexandra Adams stand outside after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton on Nov. 1, 2017.

Joe Amon, The Denver Post Police at shooting in Walmart on Nov. 1, 2017 in Thornton.



RJ Sangosti, The Denver Post Witnesses walk from a bus to Thornton Civic Center after two men were killed during a shooting inside the Walmart Super Center iNovember 1, 2017 in Thornton.

John Leyba, The Denver Post Family and friends wait on a hill to hear word from their loved ones as emergency crews and police patrol the grounds of the Walmart after a shooting at the store on Nov. 1, 2017

Helen H. Richardson, The Denver Post A father and son, who were customers inside Walmart, are escorted out of the parking lot and away from the scene of the Walmart store where a shooting occurred inside the store at 9901 Grant Street on Nov. 1, 2017 in Thornton.



Helen H. Richardson, The Denver Post Employees gather together outside away from the scene outside of the Walmart store where a shooting occurred inside the store at 9901 Grant Street on November 1, 2017 in Thornton, Colorado. Two people were killed and one taken to the hospital after a shooting that started just around 6:00pm.

Helen H. Richardson, The Denver Post Police officers help a disabled man leave the parking lot outside of the Walmart store where a shooting occurred at 9901 Grant Street on Nov. 1, 2017 in Thornton.

AAron Ontiveroz, The Denver Post A Thornton police officer gives instructions to people as they stand outside after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton on Nov. 1, 2017.



Helen H. Richardson, The Denver Post Walmart employees and customers that were inside the store wait to hear what to do from police as they stand behind police tape outside of the Walmart store where a shooting occurred Nov. 1, 2017 in Thornton.

Helen H. Richardson, The Denver Post Walmart employees and customers head away from the scene outside of the Walmart store where a shooting occurred on Nov. 1, 2017 in Thornton.

AAron Ontiveroz, The Denver Post A Thornton police officer watches as people are instructed to leave the parking lot after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton.



AAron Ontiveroz, The Denver Post People stand outside after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton on Nov. 1, 2017.

AAron Ontiveroz, The Denver Post Employees exit the parking lot after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton on Nov. 1, 2017.

AAron Ontiveroz, The Denver Post People stand outside after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton on Nov. 1, 2017.



John Leyba, The Denver Post Employees and shoppers are released from a holding area in the Walmart lot. Emergency crews and police on the grounds of the Walmart in Thornton on Nov. 1, 2017 during a shooting.

AAron Ontiveroz, The Denver Post A man in a Rascal is helped out of the parking lot as shoppers and employees gather outside after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton.

Andy Cross, The Denver Post Bystanders on a hill at 98th and Grant across the street from a shooting investigation at a Walmart in Thornton on Nov. 1, 2017.



Andy Cross, The Denver Post Police investigate the scene of a fatal shooting at the Walmart in Thornton on Nov. 1, 2017.

AAron Ontiveroz, The Denver Post People are escorted out after a gunman opened fire near the Halloween section of Walmart at 99th and Grant Street in Thornton on Nov. 1, 2017.

Helen H. Richardson, The Denver Post Onlookers watch police activity at the scene of a shooting inside the Walmart at 9901 Grant Street on Nov. 1, 2017 in Thornton.



Joe Amon, The Denver Post Police investigate the scene of a fatal shooting at the Walmart in Thornton on Nov. 1, 2017.

Joe Amon, The Denver Post Police investigate the scene of a fatal shooting at the Walmart in Thornton on Nov. 1, 2017.

Joe Amon, The Denver Post Police investigate the scene of a fatal shooting at the Walmart in Thornton on Nov. 1, 2017.



Joe Amon, The Denver Post Police investigate the scene of a fatal shooting at the Walmart Super Center in Thornton on Nov. 1, 2017.

Helen H. Richardson, The Denver Post A young girl walks through the parking lot away from the scene outside of the Walmart store where a shooting occurred on Nov. 1, 2017 in Thornton.

Helen H. Richardson, The Denver Post A father and his scared daughter, who were customers inside Walmart, are escorted out of the parking lot and away from the scene of the Walmart store where a shooting occurred on Nov. 1, 2017 in Thornton.



Helen H. Richardson, The Denver Post THORNTON, CO - OCTOBER 1 - A father and his scared daughter, who were customers inside Walmart, are escorted out of the parking lot and away from the scene of the Walmart store where a shooting occurred on November 1, 2017 in Thornton, Colorado. Two people were killed and one taken to the hospital after a shooting that started just around 6:00pm.

John Leyba, The Denver Post Adams County Sheriff on the grounds of the Walmart in Thornton on Nov. 1, 2017 talking to witnesses after a shooting inside the store.

John Leyba, The Denver Post People are taken away from the Walmart on a bus on Nov. 1, 2017 in Thornton after a shooting inside the store.



Helen H. Richardson, The Denver Post Police guard the front entrance to Walmart where a shooting occurred inside the store at 9901 Grant Street on Nov. 1, 2017 in Thornton.

Although authorities said “a few” individuals drew handguns, they posed no physical hazard to officers. But their presence “absolutely” slowed the process of determining who, and how many, suspects were involved in the shootings, said Thornton police spokesman Victor Avila.

It took more than five hours to identify the suspect, 47-year-old Scott Ostrem, who is accused in the seemingly random shootings. The problem for investigators came when they reviewed the surveillance footage and had to follow each individual with a firearm until they could eliminate them as a suspect.

“Once the building was safe enough to get into it, we started reviewing that (surveillance video) as quickly as we could,” Avila said. “That’s when we started noticing” that a number of individuals had pulled weapons. “At that point, as soon as you see that, that’s the one you try to trace through the store, only to maybe find out that’s not him, and we’re back to ground zero again, starting to look again. That’s what led to the extended time.”

He would not elaborate on whether any individuals were detained or tested for gunshot residue, or if anyone other than the suspect had fired shots during the incident. Avila said it would be hard to quantify how much more quickly the investigation could have proceeded.

“It was a very, very fluid situation, and we had to go with what was being presented at the time,” he said.

Darlene Jackson, a truck driver, said she was in the toy section of the store when she heard the gunshots. She later heard that people other than the shooter had guns, but they did not confront the killer.

“Why wouldn’t they draw their guns and shoot him?” she said.

Jackson said she owns a gun but didn’t have it with her at the time. She said she and her husband are going to a shooting range to practice firing the weapon.

The presence of armed civilians at a crime scene can potentially be either a help or a hindrance to police, said Joseph Pollini, professor and deputy chair of the Law and Police Science Department at the John Jay College of Criminal Justice in New York.

“It can work both ways,” said Pollini, who also worked 30 years as a New York City cop. “In one, you have law abiding citizens present at the scene of a shooting that could terminate it, assist in apprehending the individual. But generally as a rule, you turn to the police for that aspect. It’s not common for civilians to do the job of police, and the fact that they carry firearms can very much complicate things.”

Denver Post reporter Kirk Mitchell contributed to this report.