CHICAGO (CBS) — It has been four years since a 6-year-old Aurora boy disappeared, and police are hoping a new age-progression sketch will bring in clues on his whereabouts.

On May 11, 2011, Timmothy Pitzen’s mother picked him up from Greenman Elementary School in Aurora. Amy Fry-Pitzen told the school there had been a family emergency, and she had to take her son and leave. But there was no emergency.

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Instead, Fry-Pitzen took the boy on a trip to Brookfield Zoo, a water park in Gurnee, and finally the Wisconsin Dells, where the last known sighting of Timmothy was caught on tape, as they checked out of a resort at the Dells. Two days later, Fry-Pitzen was found dead in a Rockford hotel, and Timmothy had vanished.

Fry-Pitzen had left a suicide note indicating Timmothy was safe, and was with someone who loved him, but would never be found.

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Police have received several tips in the intervening years, but have found no sign of Timmothy. On Tuesday, the National Center for Missing & Exploited Children released the latest age-progressed image of Timmothy, showing how he would look now at age 10.

“NCMEC forensic artists do amazing work and their images have helped bring home hundreds of other children before. This important tool may hold the key to bringing Timmothy home,” said Aurora Police Det. Lee Catavu, the lead investigator on the case.

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When he went missing, Timmothy was about 4 feet 2 inches tall, and weighed around 70 pounds. He had brown hair and brown eyes. He might go by the nicknames Tim or Timmy. Anyone who might have information about his whereabouts should call The National Center for Missing & Exploited Children at 1-800-THE-LOST (1-800-843-5678) or Aurora Police at (630) 256-5500.