(CNN) -- Four suspects have been questioned in Portugal in connection with the 2007 disappearance of Madeleine McCann, a source close to the investigation told CNN on Wednesday.
The suspects, all Portuguese, were questioned by Portuguese police, with the assistance of Scotland Yard, as London's Metropolitan Police are known.
One of the suspects suffers from schizophrenia, one is a former taxi driver, and another was just 16 years old at the time of the girl's disappearance, the source said.
Madeleine was a few weeks shy of her fourth birthday when she vanished from her family's holiday apartment in the Portuguese resort town of Praia da Luz.
Her parents had left Madeleine and her younger twin siblings asleep while they went for dinner with friends at a tapas restaurant nearby. Her mother raised the alarm when she checked on the children about 10 p.m. and discovered her daughter missing.
According to the source, Scotland Yard is focusing on the four suspects because some of them made calls to one another the night of the girl's disappearance, as well as the night before and the night after.
They had asked to search the suspects' homes, but were denied by courts that found no evidence to warrant such a search, the source said.
Scotland Yard believes the four suspects called one another to plan a robbery, which went wrong when they saw the girl at the home.
British police recently finished digging in an area of wasteland close to where Madeleine went missing in Portugal. They searched two areas, according to the source, and will be leaving Portugal in the next few days.
In July 2013, Scotland Yard reopened its investigation into the little girl's disappearance after a two-year review of the original probe. Portuguese police reopened their investigation last October.
At the time, the Metropolitan Police said the two police forces' investigations would run in parallel.
Madeleine's parents, Kate and Gerry McCann, launched a massive publicity campaign to find their daughter after she went missing. They say they continue to believe she is still alive.