Twenty years later, Kimber Biggs still vividly recalls what she encountered on her Mesa, Arizona, street the day her older sister,11-year-old Mikelle Biggs, vanished.

After getting chilly — and growing annoyed that Mikelle wouldn’t give up the two-wheeler — Kimber walked home, leaving her sister alone, she tells PEOPLE in this week’s issue.

Their mother, Tracy, sent Kimber right back out to fetch Mikelle as dinnertime approached. They hadn’t been apart for even two minutes.

“So I turned around and walked to the end of the driveway and I just yelled,” recalls Kimber, now 29. “I looked both ways down the road to see where she was riding, and I didn’t see her at all.”

“At this point it was kind of dusk,” she says, “and I looked down the road and I saw something in the road but I didn’t know what it was. So I started to walk toward it and realized it was my bike.”

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The Mesa, Arizona, street from which Mikelle Biggs went missing on Jan. 2, 1999.Mesa Police Department

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Mikelle Biggs.Courtesy Biggs Family

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“At that point I didn’t totally know something was wrong,” Kimber says. “It was a very eerie feeling, almostTwilight Zone. It didn’t feel totally real.”

As Kimber drew closer to the bike that was left behind, “I noticed that the tire was still spinning.”

“That was the last time I saw her,” she says of her sister.

Police who later reconstructed Kimber’s steps concluded Mikelle was out of her sight for a virtual instant — launching an enduring mystery that is revisited in the latest episode ofPeople Magazine Investigates, airing Monday night onInvestigation Discovery. (An exclusive clip is above.)

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Kimber maintains a Facebook page,Justice for Mikelle Biggs, dedicated to the effort.

Says Jerry Gissel, a retired Mesa police investigator: “You never know when somebody is going to come forward with the information that’s going to solve this.”

People Magazine Investigates: Gone in 90 Secondsairs Monday (10 p.m. ET) on Investigation Discovery.

source: people.com