The single-storey building is heavily guarded with gun-toting policemen manning the gate. CCTV cameras constantly sweep the perimeter for possible intruders.

But the irony is unmistakable.

At the headquarters of the Madhya Pradesh special task force (STF), the team probing the state’s biggest examination scam is itself under siege, facing a potential credibility crisis with allegations of a goof up on the investigation flying from all sides.

In August 2013, the STF took over the probe into the scam in the Madhya Pradesh Professional Examination Board, also known as Vyapam from its Hindi acronym -- a multi-layered examination racket allegedly involving touts, students, job aspirants, government officials and political leaders.

The first questions about the STF’s ability – and sincerity – to conduct a fair probe were raised in 2014 when several people moved the high court.

On November 5 last year, the high court constituted a special investigation team (SIT) headed by a retired judge to monitor the STF’s probe.

What has made matters worse for investigators are a spate of mysterious deaths of people with some links the scam.

The suspicions have only grown deeper with each of those deaths.

Read: Cong wants Chouhan sacked over Vyapam scam, asks Modi to speak up

“For a scam of this magnitude, it is impossible that state-level officers can conduct proper investigation, when almost everyone at the top is involved in some way (or the other),” a retired IPS officer told Hindustan Times on condition of anonymity.

The STF itself has admitted in court that at least 23 deaths are not above suspicion but the opposition Congress has pegged the number at nearly 40.

Besides political parties, even whistleblowers and activists are pointing fingers at the STF, alleging that it has avoided action against high-profile individuals and remained soft on bureaucrats and kin of ministers whose names have cropped up in the course of the probe.

As officials of the STF are drawn from the state police, critics point out that they can be influenced (see graphic), which has led to a growing demand for a CBI probe into the scam.

Vyapam scam: No CBI probe till SC or HC asks, says Rajnath

Former legislator and one of the whistleblowers Paras Saklecha alleged that the STF investigation was just aimed at saving the high and mighty.

“It was an organised crime, the racket was functioning for years, with everybody at the top, that is bureaucrats, ministers and other high-profile persons benefiting from it. However, these have not been probed and neither top officials of (the) bureaucracy nor top politicians have been quizzed,” he added.

Sources pointed out that a former chairperson of Vyapam, Ranjana Chaudhary, was accused of earning heavy amounts from touts involved in the scam but she was not made an accused despite the allegation finding mention in the STF chargesheet.

What has also surprised many is the fact that the STF has “failed” to locate another accused Gulab Singh Kirar, known to be close to chief minister Shivraj Singh Chouhan. However, there are allegations that he has been attending public functions and even shared the dais with Chouhan a few months ago.

The retired IPS officer pointed out that it was only the CBI which managed to crack the murder case of RTI activist Shehla Masood, referring to an unrelated case which was initially probed by MP police.

 “The police had failed to quiz politicians and even tried to pass it (Masood’s murder) off as suicide as high-profile people were linked,” he added.

Congress leaders echo the same sentiment and allege that the STF works on the pick-and-choose formula, doing their best to save the high-profile and well-connected accused.

On its part, the STF has only made matters worse by cutting off all forms of communication with the media. Officers have stopped taking calls or responding to SMSs to even “friends” in the media.

STF chief Sudhir Sahi too has refused to clear the air, allowing the suspicions to grow thicker.

Whistleblower Saklecha said, “The manner in which (the) investigation is going on, even constables could have done a better job.”