How a 3-year-old helped detectives crack her mother's murder case

Lyntell Washington's daughter was only 3 years old and had just witnessed her mother's brutal murder when she was found alone, with blood on her feet, in a Louisiana parking lot in June 2016, investigators said.

In the face of the trauma, the girl would become a key witness, helping to bring her mother's killer to justice, according to investigators who followed the clues. Those clues ultimately led to the arrest and conviction of Robert Marks, Washington's secret boyfriend.

When good Samaritan Leslie Parms found the girl in the Baton Rouge parking lot, she immediately told him that "Mr. Robbie" put her there. "Mr. Robbie" was the girl's name for Marks, according to Washington's friends.

"Okay, so who made the blood? Who made the blood in the car?" asked an officer responding to the scene, according to dashcam footage.

"Mr. Robbie," Washington's daughter responded.

Marks was one of the assistant principal at Brookstown Middle Magnet Academy, where Washington worked as a teacher. The two were having an affair, and Washington was pregnant with Marks' child at the time of her death, investigators discovered. Washington's body was found a week after her daughter was left alone in the parking lot.

The girl was interviewed at a children's counseling center by a child forensic interviewer following her mother's murder. In the taped interview, the girl again said that "Mr. Robbie" made the blood in the car and gave more details.

"What did you hear when mommy got hurt? What did you hear with your ears?" the child forensic interviewer asked.

"I heard a boom," Washington's daughter said.

"Did you see Mr. Robbie hurt mommy?" the interviewer asked.

"Yes, ma'am," Washington's daughter responded.

"When we found the body, we did find that she was shot in the head. So that corroborated what the little girl said," Tony Clayton, the Iberville Parish District Attorney, told "20/20."

Prosecutors played the video of the girl's interview during Marks' murder trial. One of the jurors at the trial told "20/20" he was moved by the video.

"She was telling it. I don't think he expected that at all," the juror told "20/20."

Lyntell Washington's daughter, now 9, lives with her father, Darren Glasper. She told "20/20's" Deborah Roberts that she still remembers her mother.

Glasper told "20/20" he's been helping his daughter adjust to this major void in her life. He said he sees a lot of her mother in her.

"I see that she's smart, educated. She's focused when she puts her mind to stuff," Glasper told "20/20."