Paul Knapman is overseeing an inquest into the death of Gareth Williams, who was discovered in the bathtub of his London home on Aug. 23. Police have made no arrests and are not certain how the 31-year-old employee of code-breaking agency GCHQ died.
At an inquest hearing Tuesday, Knapman said it was "less and less likely" that a man and woman seen at Williams' home weeks before his death would be found.
The couple, described as in Mediterranean appearance and aged between 20 and 30, were seen at the door of Williams' apartment block in June or July, and indicated to a witness that they had a key.
Knapman said police had to face the fact that "in six months or so they have not come forward" and no new clues have emerged.
Williams' death has spawned fevered speculation and conspiracy theories, as police have tried to determine whether it was linked to his work or to his personal life.
Detectives have said inquiries showed Williams had viewed bondage-related websites, and have suggested he may have died in a sex game gone wrong.
Security officials have downplayed suggestions that Williams' death was connected to his work. He was working on attachment to the foreign spy service, MI6, when he died.
Toxicology tests found no traces of drugs or poisons that may have led to his death. Experts consulted by police said Williams could not have locked himself inside the bag - which was fastened with a padlock - and could have survived for only 30 minutes inside before suffocating.
In Britain, inquests must be held any time someone dies unexpectedly, violently or from unknown causes. The goal is to determine the cause rather than to affix blame.
Knapman said about 40 of Williams' fellow spies had been interviewed by police and could be called to give evidence anonymously at the inquest.
He adjourned the hearings until March 31, so police can get results from more forensic tests.