Exclusive: Shop employees say they may have sold costume to alleged 'Killer Clown' murderer

— -- More than 27 years ago, a woman died after she was shot at her front door by someone dressed as a clown, and a former costume store employee says there is a “strong possibility” that the woman’s accused murderer bought the outfit at her shop.

On the morning of May 26, 1990, Marlene Warren was at her home in Wellington, Florida, with her 22-year-old son, Joseph Ahrens, and several of his friends.

Ahrens and his friends told police they saw a white car pull into the driveway and that a person in a clown costume came to the front door with flowers and balloons.

Warren answered the door, and as the clown offered the items to her, witnesses told police they heard a gunshot. Warren had been shot in the face. She ultimately died from her injuries.

The clown then “calmly” walked back to the car and drove away, police say.

Detectives investigating the murder spoke to Deborah Offord and Barbara Castricone, who used to work at a costume shop in town.

"I get goosebumps just talking about it," Castricone told ABC News' "20/20."

They recall that a day or two before the murder, a last-minute customer arrived after 6 p.m., when the door to the store was locked.

“I need to buy a costume,” the customer said, according to Offord, who then replied, "Can you come back tomorrow, we're closed."

"No, I really need this tonight. I need to get a costume tonight,” Offord remembered the customer saying.

Offord said the customer also insisted that the costume had to be circus wear.

“A Rubie’s clown costume, an Afro clown wig, Bob Kelly clown makeup and a sponge nose,” said Offord.

Shortly after the shooting, a detective called and started asking questions about the sale.

Castricone said she was shocked when a detective told her why they were asking about the mysterious customer.

“I said, ‘Could I ask please what this is in reference to?’ He said, ‘Well, you're probably going to read about it all over the papers tomorrow morning, but a woman was shot at her front door by a person dressed in a clown costume bearing flowers and balloons, and [the clown] shot her in the face.’ And my heart dropped,” Castricone said.

Offord offered cops a description of the female customer.

“Rather tall, I’d say probably around 5’8, brown eyes, long chocolate hair, thick head of hair, work boots,” said Offord. “I don’t know why I remember that. I do, and jeans, and a men’s work shirt, a button down collar.”

Castricone said the customer paid for the sale in cash, using $20 bills. Her store was only one of a handful in the area that sold costumes at the time.

The investigation into who murdered Warren eventually went cold, until 2014.

As a result of the ensuing investigation, police found and arrested Sheila Keen Warren on Sept. 27, 2017, in Virginia. Twelve years after Marlene Warren’s death, Sheila Keen had married Marlene Warren’s husband, Michael Warren.

Sheila Keen Warren was charged with first-degree murder, and prosecutors say they are seeking the death penalty.

Her attorney, Richard Lubin, said Warren "vehemently denies" killing Marlene Warren and will plead not guilty, The Associated Press reported.

Lubin declined to comment to ABC News.

Michael Warren recently told ABC News, “She’s is falsely accused…[and] this is very serious and very unfair.”

Watch the full story at ABC News' "20/20" on Friday at 10 p.m. ET.