Like many gun store owners these days, Gregory Isabella spent the night in his South Philly shop on guard after previous break-in attempts.

He was ready when, about 4:15 a.m. Tuesday, a group of four men cut the lock and kicked in the door at Firing Line Inc. on the 1500 block of South Front Street. Isabella, 67, told police he was on the second floor when he heard them approach. He then shot and killed one of the alleged looters.

By day, hundreds of residents wait in long lines for hours to buy guns, often their first. By night, gun shop owners say they are the new targets of burglars and thieves.

Isabella "heard them walking up the steps, and one of the individuals who broke into the property pointed a handgun at him,” Philadelphia Police Inspector Scott Small told Fox 29. “And that’s when the store owner fired his own weapon, striking the one perpetrator at least one time in the head.”

Police said the weapon Isabella discharged was a Bushmaster M4 rifle. They also said officers recovered a black handgun believed to be carried by the man Isabella shot.

Medics pronounced the man dead at the scene, and the other individuals ran away, Small said. Isabella was not injured. Police did not identify the dead man.

Reached by phone Tuesday, Isabella told a reporter, “No comment. Don’t call here again,” and hung up.

» READ MORE: Here's live coverage of what's happening Tuesday

Small said an individual arrived at Thomas Jefferson University Hospital with a gunshot wound to his shoulder shortly after the incident, and said it was a “possibility” the person was connected to the burglary attempt.

No arrest was made, but an investigation is ongoing, according to police. Assistant District Attorney Joanne Pescatore was spotted at the crime scene.

During a news conference Tuesday afternoon, Mayor Jim Kenney and Police Commissioner Danielle Outlaw said that while they respect the rights of business owners to protect their property, they were disturbed to hear that a business owner shot and killed a potential burglar.

“We do not endorse or condone any form of vigilante justice or taking the law into one’s own hands,” Outlaw said.

Kenney described himself as “deeply troubled” by the death.

But across the city, gun store owners say they are sleeping inside their shops after break-in attempts.

William H., who did not want his last name used, saying he feared becoming an “antifa target,” owns Founding Fathers Outfitters on Ridge Pike in Montgomery County. He, and usually another employee, have slept in his store since early Sunday, when, he said, six people tried to break into his gun shop.

At 12:59 a.m. Sunday, his surveillance cameras captured video of six men, all in masks, trying to break in. One had a baseball bat and tried to bash in the front glass door. The glass shattered in a spider-web-like shape, but did not break. A man behind him held a hammer.

“They wanted to do a smash and grab,” William H. said. “Take a hammer and smash the cases and grab what they wanted. Unfortunately for them, we lock every firearm in a safe.” They never got in the store.

From then on, William H., 37, who said he was a flight medic in the Army for 13 years and then worked as an EMS chief, has spent the night in an office chair or on the floor of the shop. “Depending on my back,” he said.

Patricia, his salesperson, said she prefers a sleeping bag. They are armed with pepper spray, Tasers, flash bangs, a grenade that disorients people with a bright flash and loud noise, rubber bullets and guns.

“We don’t want our firearms to get in the hands of criminals. We’ll do anything to stop that from happening,” said Patricia, 22, who is also an EMT and also said she did not want her full name published for fear of retaliation.

“I’m not looking to shoot anyone. That’s the last thing I want to do,” William H. said.

He said Isabella’s having to shoot and kill someone was “unfortunate.”

“No one wants to have to do it,” he said. “I know a lot of gun shop owners and everyone is sleeping there [in their stores]. Looters are not just after the sneakers and the flat screen TVs anymore. They are getting more aggressive. They’re trying to get the guns.”

On Tuesday, William H. opened his store at 1 p.m. instead of the usual 10 a.m. He needed rest. But by 12:30 p.m., there was a line of about 50 people in surgical masks or bandannas wanting to buy guns. The broken window was covered with plywood.

The interest in guns, he said, surged as the coronavirus lockdowns were imposed.

“From the lockdown until the first week in April, basically three weeks, we did a year’s worth of sales,” William H. said.

Many are first-time buyers.

Sharletta Hargrove, 26, went to Founding Fathers on Tuesday with her 3-year-old daughter, who brought her two Barbie dolls.

They live in Clifton Heights, Delaware County, and Hargrove’s mother lives in West Philadelphia, where lots of stores have been looted and burned, she said.

“You don’t know what to expect, and I’m scared,” she said. “I just have to protect my house and family.”

Hargrove had never bought a gun before. Nor had Gary Williams, 28, who sat nearby on a tree stump waiting for the store to open.

Williams lives in Germantown with his girlfriend and two daughters, ages 1 and 5. In his neighborhood, many stores, including pharmacies and food markets, have been looted and are now boarded up. “There’s nothing available. I had a hard time finding milk and Pampers," said Williams, who works as a home health aide.

“I need to protect my family. I want the biggest gun I can find,” he said. “There are a lot of peaceful people out there, but the looters are taking advantage of good people who own their own businesses. What they don’t realize is they’re destroying their own community. I want to feel safe and secure."

It took almost six hours. But Williams stood in line, waited to get approved through the state background check process and picked out what he wanted. He walked out with an AR-12 shotgun.

“I feel 100% better," he said Tuesday night. “It’s a big relief to know I can protect my family.”