British detectives are back on the trail of a Cold War-era killer who used a poison-tipped umbrella to slay a communist defector.

British detectives acknowledged Friday that they had questioned suspects in the 1978 death of Bulgarian dissident Georgi Markov. The playwright and broadcaster was a stern critic of his country's communist regime in reports for the British Broadcasting Corp. and Radio Free Europe.

Markov was jabbed in the thigh with an umbrella tip as he waited for a bus on London's Waterloo Bridge. He developed a fever and died three days later. British government scientists later discovered the umbrella had been used to inject a pinhead-sized pellet of the poison ricin into Markov's leg.

Though no one has ever been charged with the killing, many suspected the KGB and Bulgarian secret police of involvement. KGB defector Oleg Gordievsky has previously said Russian authorities offered help to Bulgaria for the murder plot.

The case remained one of the most remarkable espionage-related deaths in London until the killing of ex-Russian security agent Alexander Litvinenko in November 2006. Litvinenko, a fierce Kremlin critic, died after he ingested the radioactive element, polonium-210, most likely from a cup of tea laced with the poison.

Police in London said the Markov case has never been closed, and that officers are following up a raft of new leads.

A small team of officers from London's Metropolitan Police went to Bulgaria in May in connection with the inquiry into Markov's death, a police spokeswoman said Friday on condition of anonymity in line with police department policy. She said the inquiry remains open and has been a particularly complex investigation.

Officers also visited Bulgaria in March and in April last year to review files and request access to witnesses, including Communist-era secret police officers.

Though Markov's killer has never been charged, Vladimir Todorov, a former intelligence chief, was jailed in Bulgaria in 1992 on charges of destroying files related to the case.

Andrei Tsvetanov, the Bulgarian investigator in charge of the case was quoted by Bulgaria's Dnevnik daily as saying, "We are offering our full cooperation to our British colleagues and I can assure that now we have a 100 percent exchange of information on both sides — something we lacked so far."