(CNN) -- It's been more than six years since Madeleine McCann vanished while on holiday with her parents in Portugal.
But neither her parents nor the detectives investigating her case are giving up on one day finding the little girl from Leicestershire, England.
Her disappearance is set to get renewed attention Monday when investigators make another appeal to the public on BBC's "Crimewatch," releasing two computer-generated sketches of a man spotted around the resort town of Praia da Luz on the day then-3-year-old Madeleine went missing. The sketches are based on descriptions from two separate witnesses.
"Today I am asking the public for their help. Whilst this man may or may not be the key to unlocking this investigation, tracing and speaking to him is of vital importance to us," Detective Chief Inspector Andy Redwood said in a statement Monday before the program, which will air in the United Kingdom, the Netherlands and Germany. "We have witnesses placing him in the resort area around the time of Madeleine's disappearance."
The man is described as white, between 20 and 40 years old, with short brown hair and a medium build. He was seen carrying a blond child, who may have been in pajamas and who was estimated at 3 to 4 years old, around the time Madeleine disappeared, police said Monday.
"There may be an entirely innocent explanation of this man, but we need to establish who he is to assist with our inquiries," Redwood said.
Additionally, investigators believe they have a better understanding of when Madeleine may have been abducted, Redwood said. The "Crimewatch" program Monday night will reconstruct the events of May 3, 2007 -- the night Madeleine disappeared from her family's villa while her parents dined at a nearby restaurant. She was just days shy of her fourth birthday.
"The timeline we have now established has given new significance to sightings and movements of people in and around Praia da Luz at the time of Madeleine's disappearance," Redwood said.
Police announced in July that they have identified 38 "people of interest" in connection with the case. Twelve of them are UK nationals who police say they think were in Portugal at the time the girl went missing. All the others are European nationals.
Metropolitan Police officers are talking to authorities in other countries to get more information about them and their potential involvement, the statement said.
Meanwhile, Madeleine's parents continue to keep hope that their daughter will be returned to them, explaining on their website: "We still worry about her, we miss her as much as we ever did, we remain as determined as ever to find her and to know what has happened."