WELLINGTON, FLORIDA: The son of a victim who was murdered by a killer clown on May 26, 1990, has recalled the tragic moment when he saw his own mother get fatally shot in front of his eyes.

On the morning of that day, Marlene Warren was having breakfast with her son Joe Ahrens in their home in Florida when he said a clown came to their front door with a bouquet of flowers and two balloons.

Ahrens remembered his mother happily answering the door of their home in Wellington, Florida, which is about 30 minutes west of West Palm Beach.

He said she accepted the apparent gifts and commented, "How pretty", but little did he know that it would be his mother's final words.

How was Marlene Warren killed?

According to the authorities, the clown raised a gun within seconds and fatally shot Marlene in the head, ending her life, prompting a decades-long search for her killer.

"It took over 30 years to make sense of it all," Ahrens told CBS News correspondent Peter Van Sant.

"Marlene Warren was someone without any known enemies," said Palm Beach County State Attorney Dave Aronberg.

"This was an assassination. This was not a random act of violence," Aronberg added.

Following Marlene's murder, investigators started to check who had a motive to kill the mother and small business owner.

Who killed Marlene Warren?

The police quickly traced down her husband, Michael Warren, who was also Ahrens' stepfather. Ahrens said his mother had warned him to be suspicious of Michael.

"She said, 'if anything does happen to me, your father did it,'" said Ahrens, adding "I told her, 'No way. He would never do anything like that.' She said, 'Don't put it past him.'"

Besides, investigators also found a possible motive. Michael was apparently involved in a romantic relationship with his co-worker, Sheila Keen, who repossessed cars at the auto business he ran.

"In the hours after the murder of Marlene Warren, the suspects became clear," said Aronberg, adding "It was Sheila Keen, and it was Michael Warren, but Michael Warren had an alibi."

Authorities later confirmed that Michael was on his way to a horse race with his friends at the time Marlene was shot. As a result, he was ruled out as the shooter.

Where was Sheila Keen at the time of Marlene Warren's murder?

When investigators interrogated Sheila about her whereabouts at the time of Marlene's murder, her responses were not concrete.

She claimed she was out looking for cars to repossess, however, she could not verify her exact location.

Detectives also learned that before the murder a woman fitting Sheila's general description was spotted purchasing a clown costume, as well as the balloons and flowers left by the clown.

When detectives found a car fitting the description given by Ahrens of the getaway vehicle, they said they found a brown hair that was visually consistent with Sheila's hair.

They also found orange fibers which matched the description of the wig the clown was wearing inside Sheila's car and in her apartment.

"Sheila Keen-Warren had the means, the motive, and the opportunity to do this," said Aronberg.

Sheila Keen was not charged at the time

Despite the circumstantial case building against Keen, Aronberg, who did not become the Palm Beach County state attorney until 2012, aid prosecutors at the time did not believe they had enough evidence to charge Sheila with Marlene's murder. So the case grew cold as time passed on and there was no new evidence. "For many years … I was suffering in, in despair," Ahrens said.

Sheila Keen got married to Michael Warren

A cold case unit retested old evidence in 2017 and determined that Sheila's DNA matched the hair found in the suspected getaway car. However, when investigators started looking for her, they learned she had taken on a new moniker.

Almost 12 years after the murder in 2002, Sheila had wed Michael Warren and was now Sheila Keen-Warren. "Here's someone whose wife had been murdered and he just married the chief suspect," said Aronberg.

"When you combine the fact that the two of them were in an affair … and then later, they got married, it did seem like mission accomplished," he added.

The couple had relocated to Kingsport, Tennessee, where they had been running a burger joint called Purple Cow for years.

Detectives spoke to one employee who said one year at the restaurant Sheila had worn a clown costume for Halloween. The worker even provided detectives with a picture of her dressed up as a clown.

When was Sheila Keen-Warren finally arrested?

On September 26, 2017, 27 years after the shooting Sheila Keen-Warren was arrested for murdering Marlene Warren.

"She probably never thought in a million years she'd be held accountable for her crimes," said Aronberg about the arrest, adding "She thought she got away with it."

The 59-year-old pleaded guilty to second-degree murder on April 25 this year in a deal that will likely see her released from prison in no more than two years.

Palm Beach County State Attorney Dave Aronberg said in a statement that the plea deal “obtained a measure of justice” for Marlene Warren and her son.

What happened to Michael Warren?

Michael also had a long past of run-ins with law enforcement. He was convicted in 1994 of grand theft, racketeering and odometer tampering for which he served almost four years in prison.

At the time, his then-attorneys said his punishment was disproportionately long because of suspicions he was involved in Marlene's murder.