Portuguese police are questioning four people in relation to the investigation into the disappearance of Madeleine McCann.
The men, pictured arriving at a police station in Faro on Tuesday, have been identified by Scotland Yard detectives as being able to assist them in their search.
Two of the men, believed to be suspects, are part of a group of 'arguidos', or persons of interest, who will be interviewed by police.
One of the men is believed to be Sergey Malinka, a 29-year-old Russian-born IT engineer who was first questioned by police as a witness in 2007, though never declared a suspect.
A law office representing Malinka confirmed to The Guardian that he was being questioned and the "complicated" process "was likely to continue into Wednesday."
Malinka, who has Portuguese nationality, has always denied any involvement in Madeleine’s disappearance from Praia da Luz.
It is believed he was questioned in 2007 because of his business links with British expat Robert Murat.
Murat was declared a suspect but later cleared and received more than £600,000 in libel damages from 11 British newspapers. Malinka also received undisclosed damages.
It is thought that UK officers are sitting in on the interviews, which are believed to be taking place in Faro, but will not ask questions themselves.
Last month British officers, accompanied by their Portuguese counterparts, carried out searches of three areas of land near to where the young girl went missing.
She had been in a holiday apartment as her parents Gerry and Kate, from Rothley in Leicestershire, dined with friends nearby.
On finishing the latest search police said it had been the ''first phase of this major investigation which has been agreed with the Portuguese''.
A statement from Scotland Yard during the searches said there was "still a substantial amount of work yet to be completed in the coming weeks and months''.
''This recent work is part of ensuring that all lines of inquiry are progressed in a systematic manner and covers just the one hypothesis that she was killed and buried locally,'' the statement said.
''This is the same as would be done in the UK for a murder or high-risk missing person inquiry.''