LONDON (Reuters) - Portuguese police investigating the disappearance of British four-year-old Madeleine McCann acknowledged on Saturday that she might be dead.

Chief Inspector Olegario Sousa, one of the detectives leading the inquiry, told the BBC that recent evidence had prompted officers to pursue more intensely the theory she might have been killed.

Madeleine, from Rothley, Leicestershire, disappeared on May 3 during a holiday with her family in the resort of Praia da Luz, in the Algarve region of southern Portugal.

"In the past few days there have been some developments and clues that have been found that could point to the possible death of this child," Sousa told the BBC in an interview.

"All lines had been open but this line is now checking with a little bit of intensity," he said, speaking in English.

The clues to which he was referring were traces of blood found inside the apartment where Madeleine was sleeping, at the Mark Warner Ocean holiday resort, the BBC said on its Web site.

"We are waiting for lab results that have been collected," he added.

The traces of blood were being analyzed in Britain to find out whether they matched Madeleine's.

Sousa refused to confirm or deny reports that police sniffer dogs had detected odors of a dead body in the apartment.

It was the first public acknowledgment by the police that Madeleine might have died, rather than been abducted.

He said her parents, Gerry and Kate McCann, were not considered suspects, following media speculation that they were under suspicion. Neither were friends who had been with them on the night Madeleine disappeared, he said.

The family marked the 100th day since her disappearance with a special church service on Saturday in Praia da Luz.

Despite the publicity, so far only one suspect has emerged, Briton Robert Murat, while sightings and reported breakthroughs have come to nothing or proved to be hoaxes.

The McCanns launched "Don't You Forget about Me", a channel on the video-sharing Web site YouTube to help trace missing children, to mark 100 days without their daughter.