* "Deep state" allegedly behind political killings
* Prosecutors investigate series of suspicious deaths
* Ozal autopsy reveals poison, cause of death unclear (Recasts with Malatya prosecutors' probe, background)
By Daren Butler
ISTANBUL, Dec 12 (Reuters) - Turkish prosecutors are investigating the 1990s deaths of public figures including former President Turget Ozal, a newspaper said on Wednesday, acting on long-held suspicions they may have been killed by a covert establishment network.
State prosecutors in the eastern city of Malatya have widened their probe into the 1994 death of a colonel, recorded as committing suicide but suspected to have been murdered, and have requested state dossiers on the deaths of Ozal and three other public figures, the Haberturk newspaper said.
It said that the prosecutors' office believed the four may have been targeted over attempts to end Turkey's conflict with the Kurdistan Workers Party (PKK) militant group, which took up arms against the state in 1984.
A separate forensic institute autopsy on the exhumed body of Ozal, who led Turkey out of military rule in the 1980s, was completed on Tuesday. While it revealed traces of poison, the actual cause of death was unclear, media reports said.
"In my view, the forensic institute report has not eliminated the suspicions," Deputy Prime Minister Bekir Bozdag told reporters in parliament on Wednesday, saying the investigation was continuing.
"If there is poison in the body where did it come from?"
There have long been rumours that Ozal, who died of heart failure in 1993 aged 65, was murdered by an alleged organisation within the Turkish establishment in which military and security officials supposedly colluded with criminal elements.
The "deep state" is believed by many in Turkey to have been a clandestine network set up by nationalist groups operating in the shadows and murdering anyone deemed to pose a threat.
Ozal, whose economic reforms helped shape modern Turkey, was in poor health before his death. After undergoing a triple heart bypass operation in the United States in 1987, he kept up a gruelling schedule and remained overweight until he died.
His moves to end a Kurdish insurgency and create a Turkic union with central Asian states have been cited as motives for would-be enemies and he survived an assassination bid in 1998.
Prime Minister Tayyip Erdogan has often characterised his decade in power as a battle against anti-democratic forces, rhetoric his critics say is a pretext to stifle opposition.
But while he has reined in a once all-powerful military that ousted a series of governments, the activities of the "deep state" have remained shrouded in mystery.
The investigations come as a trial of suspected members of "Ergenekon", an alleged underground network of nationalist coup plotters often linked to the "deep state", approaches its conclusion at a court in Silivri near Istanbul.
Thousands of people are believed to have been killed in extra-judicial and unsolved killings in Turkey in the 1990s, mostly linked to the conflict with the PKK in mainly Kurdish southeast Turkey, according to human rights groups.
Among the four people whose deaths the Malatya prosecutors are looking into are a former commander of Turkey's paramilitary gendarmerie force, Esref Bitlis, who died in a small plane crash minutes after taking off in February 1993.
His death is already the subject of a probe by prosecutors in Ankara investigating sabotage allegations. A defendant in the Ergenekon trial has been questioned over the incident.
The Malatya prosecutors also sought dossiers on Ozal adviser and finance minister Adnan Kahveci, who died in a February 1993 car crash, and Diyarbakir gendarmerie chief Bahtiyar Aydin who was assassinated in October 1993 in an attack blamed on the PKK.
Previous media reports have said Ozal's body, dug up in October on the orders of prosecutors, revealed traces of insecticides, pesticides and radioactive elements. (Writing by Daren Butler; Editing by Nick Tattersall and Alison Williams)