London, England (CNN) -- Metropolitan police in London, England, appealed Monday for further information about last month's death of a man who worked for a British intelligence agency and whose naked body was found in a padlocked duffel bag in his bathroom.

Police also released surveillance camera images of Gareth Williams taken August 15, about a week before officials discovered his body on August 23.

In addition, police said they would like help identifying "a man and a woman, both of Mediterranean appearance," between the ages of 20 and 30 and who were seen entering Wiliams' apartment building late one evening in June or July.

"This remains a complex unexplained death enquiry," Det. Chief Inspector Jacqueline Sebire said in a statement Monday.

Williams worked at Government Communications Headquarters (GCHQ), Britain's intelligence agency for monitoring communication and keeping government data secret, the agency told CNN. He was widely reported to have been on loan to MI6, the foreign intelligence service, in London.

The crime scene did not seem to reveal much information.

"There was no sign of any forced entry to the property, and no signs of disturbance inside," police said, adding that robbery did not appear to be a motive.

"We do not believe there is any property missing from the flat," the police statement said. "There is no suggestion the items within the flat were specifically posed. No drugs, or indications of drug usage were recovered."

An August 25 autopsy "established no obvious cause of death," police said.

"Extensive forensic tests at the flat in Alderney Street and from the samples taken at the post mortem continue," police said.

"Initial indications from the toxicology examinations show no trace of any alcohol or routine or recreational drugs. Testing for any other substance continues."

Police believe Williams was last seen alive on August 15, when surveillance cameras show him shopping in the West End and Knightsbridge areas of London.

He had returned August 11 from a trip to the United States, police said Monday.