What to Know Maurice Louis purchased a shotgun from a Philly gun shop the same day he allegedly shot to death his mother, stepfather and 2 half-brothers.

Police described Louis as "mentally unstable." He was allegedly found naked and drinking vodka in the house where police found the victims.

The gun shop owner told NBC10 that Louis "seemed perfectly A-okay" when he bought the shotgun.

A firearms dealer that has operated in Northeast Philadelphia for more than 60 years sold a shotgun Tuesday to the man suspected of killing four of his family members later that same day.

Maurice Louis, 29, purchased a Mossberg Maverick 88 from Lock's Gun Exchange, the gun shop's owner confirmed Thursday. Louis then took the SEPTA Market-Frankford elevated train line to his family's house in West Philadelphia, where he allegedly used the shotgun to kill four immediate family members, according to police.

Responding officers found the family after Louis's parents failed to show for work, and found Louis naked and drinking vodka from a bottle in a second-floor bedroom, according to police sources. Police described him as "mentally unstable."

"My heart is down around my ankles," Virginia Lock, owner of Lock's Gun Exchange, told NBC10 in a phone interview.

Lock said her shop on Rowland Avenue performed the required Pennsylvania Instant Check System search on Louis.

"We do everything by the books," Lock said.

Louis has been charged with four counts of murder for the shooting deaths of his mother, 51-year-old Janet Woodson, his step-father, 56-year-old Les Holmes, and his two half-brothers, 18-year-old Sy-eed Woodson and 7-year-old Leslie Holmes.

Officers found the four dead inside their home on Walton Street near the Cobbs Creek section of West Philadelphia.

A receipt for the purchase of the shotgun from Lock's Tuesday was found by detectives at the scene, according to a police source.

Purchasing a firearm in Pennsylvania only requires that the buyer pass a background check, which can be completed in minutes at the time of sale.

"If an individual is eligible to acquire a firearm, the PICS background check replaces the former, mandatory five-day waiting period," according to the Pennsylvania State Police website. "Operation has shown that approximately 60% of the individuals attempting to purchase a firearm are approved within minutes."

Anyone over 21 can buy a firearm from a licensed dealer in Pennsylvania, with a few exceptions. Those are people who have: violent crime convictions, undocumented immigrant status, been declared mentally ill by the court, fugitive status, active protection from abuse orders or three separate DUI charges within a five-year period.

Police described Louis as "mentally unstable" after his arrest Wednesday. They did not give further details.

Family members, who spoke with NBC10 on Wednesday night, said Louis has struggled with mental illness.

"There is no way we can know if a person is mentally ill or not," Virginia Lock, the gun shop owner, said. "He seemed perfectly A-okay when he was here."

It's the worst violence she said she can remember being associated with a purchase from her shop in its 63-year existence.

"I mean, we're just devastated," Lock added.