The parents of Madeleine McCann fear Portuguese detectives are about to name them as suspects in their daughter's disappearance, a family friend said yesterday.

It followed Kate McCann's return to an Algarve police station to undergo questioning as a witness for a second time.Her husband Gerry will be re-interviewed today.

Before entering the headquarters in Portimao at 2pm, Mrs McCann issued an appeal to Madeleine's abductors to "do the right thing".

She said: "I miss Madeleine so much. Gerry and I want to appeal again to the person, or people, who took her, or know who took her, to do the right thing. It is not too late - please let her go or call the police."

Mrs McCann was still being interviewed well into the night. Portuguese police have not revealed why they wanted to speak to the couple again. But there is speculation the move may be linked to forensic tests on samples taken from the McCanns' holiday apartment in Praia da Luz, from where four-year-old Madeleine went missing while her parents dined with friends at a nearby restaurant.

The samples, believed to be blood splashed on the bedroom wall, were only detected a month ago by sniffer dogs brought in by British police, and were being analysed by forensic scientists in the UK. The interviews come amid speculation that there had been a forensic breakthrough in the case, and arrests were now imminent.

A source said information from the Forensic Science Service in Birmingham had been returned to Portuguese investigators.

A family friend of the McCanns said yesterday they regarded the interviews as a "step forward in the investigation" but were apprehensive about what they might mean. "They do fear they are about to become suspects," the friend added.

Mrs McCann, a 39-year-old GP, has only been formally interviewed by police once before - on May 4, the day after Madeleine went missing.

Portuguese detectives have questioned her Glasgow-born husband, a 39-year-old cardiologist, on two occasions.

Portuguese police have repeatedly said that the couple are not suspects in the case, but they have endured a series of media reports alleging they were involved.

The McCanns' lawyer, Lisbon-based Carlos Pintode Abreu, emerged from the police station at about 9.35pm. He walked down the street, pausing briefly to tell waiting reporters that Mrs McCann was still inside.