Relisha Rudd and her mother, Shamika Young, were staying in a homeless shelter in Washington DC. The 8 year-old-girl stopped attending school in February 2014, but it was a month before the alarm was raised.

Shamika had been befriended by Khalil Tatum, a 51-year-old janitor at the shelter. Tatum had a felony record for burglary, larceny, and breaking-and-entering. He was known for inappropriate relationships with shelter residents, and for paying particular attention to young girls.
Shamika allowed Tatum to take Relisha away overnight, he also brought her a laptop.

Her mother provided a note to the school claiming the child was having health problems and was in the care of a “Dr. Tatum.”
The school contacted Tatum at the number provided, but when he failed to show up for a meeting on the 19 March, a counselor contacted the police to report Relisha missing.

During the investigation, police found CCTV footage of Tatum and Relisha from the 26 February and 1 March which showed the pair walking the hallways of two different hotels.
The 1 March footage is the last known sighting of Relisha.

On the same day she was reported missing, Tatum’s wife was found shot in the head in a motel in Maryland.
On 2 March, a day after Tatum was caught on CCTV with Relisha, he was seen purchasing a shovel, lime and 42-gallon trash bags.
On 31 March, Tatum’s body was found in a shed in Kenilworth Park, dead of apparent suicide.

No trace of Relisha has ever been found. Police believe he either killed her or sold her to sex-traffickers.
In an interview with The Washing Post, a senior law-enforcement official suspected that Tatum had been sexually exploiting Relisha and possibly pimping her to others. He may have killed his wife due to her finding out about his activities.
Five years later, her step father holds out hope she is still alive, D.C. police said they’ve never given up trying to find her.

Crews of LE and volunteers have looked for evidence along the Anacostia River and in 2018 December, investigators spent days searching service tunnels beneath D.C. General.
DC officials did carry out a report on the investigation into Relisha’s disappearance. Their report determined that the District could not have prevented the little girl going missing, citing, in part, the misleading information provided by Relisha’s family. However, the report did suggest more than two dozen policy changes on matters including how schools deal with unexcused absences, background checks for homeless shelter employees and fraternization between families and shelter staff.

The General DC homeless shelter, where Relisha went missing from, was closed by Mayor Muriel Bowser in 2018.
Source
Washington Post LE quote-
https://www.washingtonpost.com/opinions/an-8-year-old-girl-in-dc-tragically-disappeared-what-happened-to-her/2016/10/07/d63704a8-8bef-11e6-875e-2c1bfe943b66_story.html?utm_term=.ccc8d80d680f