Nobody knows exactly when or why the witnesses and small-time crooks caught up in one of India's biggest-ever corruption scandals began dying under mysterious circumstances. But in the past two years, that's what's happened to more than two dozen people implicated in a US$1 billion ($1.5 billion) test-rigging scheme.

Police say that since 2007, tens of thousands of students and job aspirants have paid hefty bribes to middlemen, bureaucrats and politicians in the central state of Madhya Pradesh to rig test results for medical schools and government jobs.

So far, 1930 people have been arrested and more than 500 are on the run. Hundreds of medical students are in prison - along with several bureaucrats and the state's education minister. Even the governor has been implicated.

Police have had their hands full racing to meet a July deadline in the criminal probe. And now they are faced with the deaths of more witnesses and suspects.

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The state's government, run by the Bharatiya Janata Party, has said that "no conspiracy was found" in the recent deaths. But others involved in the case fear otherwise.

Cheating on school and college tests is commonplace in India, but in Madhya Pradesh, cheating on tests became a sophisticated racket using various approaches. High-scoring students toting fake identity cards were brought in to impersonate applicants taking tests. Applicants would be told to leave their answer sheets blank so that scorers could fill them out in a separate room. And scores would be manipulated by testing board officials to favour bribe-paying candidates.

In July 2013, after a tip-off from whistleblowers, police raided a test centre in Indore and arrested eight impersonators taking the medical school test. Police say that the scheme was operated by a syndicate of agents, doctors, officials and politicians across five states and that there was no single ringleader.

- Bloomberg