Gareth Williams' body was discovered in a holdall in the flat in Pimlico Further tests are to be carried out on the body of an MI6 worker which was found in a holdall in the bath at his central London flat.
The body of Gareth Williams, 30, from Anglesey, was found in the Pimlico flat on Monday afternoon. Police believe he may have been murdered two weeks ago.
A post-mortem examination carried out on Wednesday proved inconclusive.
The Metropolitan Police said more tests, including toxicological analysis of his blood, would be carried out.
The force is treating the death as "suspicious and unexplained".
The body was discovered when officers broke into the flat in Alderney Street after colleagues said Mr Williams had not been seen for at least 10 days.
Officers went to the flat after attempts by the Foreign Office to locate him via his former landlady failed.
'Lovely guy'
The post-mortem examination by a Home Office pathologist failed to determine a cause of death, although it is believed he was not stabbed.
Originally from Holyhead, north Wales, Mr Williams was on secondment to MI6 from his job as a communications officer at the GCHQ "listening post" in Cheltenham, Gloucestershire.
The Foreign Office contacted his former landlady Jenny Elliott on Monday after his work colleagues reported not seeing him for "some time".
Mrs Elliott said he lived in a flat attached to her Cheltenham property for 10 years and was preparing to return on 3 September.
"He phoned me a few weeks ago to say he was coming back," she said.
She described him as "a lovely guy, very friendly, very well-mannered and polite and no trouble at all".
She added: "He was often away. He went to America to work a lot and often combined it with holiday because he hated flying."
Officers who entered the Pimlico flat, which is about half a mile from the headquarters of the Secret Intelligence Service (MI6) on the banks of the Thames, discovered a mobile phone and several mobile phone Sim cards laid out.
On Wednesday, Mr Williams' uncle, William Hughes, told BBC News that his nephew never spoke about his work but he said he was "very, very talented".
His father Ian, who works at Wylfa nuclear power station and lives in Valley, Anglesey, went straight to London after being told of the death, with his wife and daughter Ceri, who lives near Wrexham.
The police are understood to be looking into aspects of Mr Williams' personal life.
It is not known what work he was doing for MI6.
After it emerged he had been working for the intelligence services, a spokesman for the British Foreign and Commonwealth Office, which is handling press inquiries for MI6, said: "This is a police matter.
"It is long-standing Her Majesty's Government policy not to confirm or deny any individual working for the intelligence agencies."
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