UPDATED 03/23/12 11:43 a.m.

JOLIET, Ill. (CBS) — New questions are being raised in the case of a father from Oswego who stands charged with killing his wife and three children in 2007.

As CBS 2’s Susanna Song reports, defense attorneys for Christopher Vaughn, 37, are now calling for independent investigators to look into allegations of police misconduct in the case.

A pre-trial hearing on the case was set for midday Friday at the Will County courthouse in Joliet.

Vaughn’s trial is set to start in August.

LISTEN: WBBM Newsradio’s Mike Krauser reports

On June 14, 2007, Vaughn; his wife, Kimberly; and their three children – Abigayle, 12; Cassandra, 11; and Blake, 8, were on their way to a water park in Springfield from their home in Oswego, when police say Vaughn shot pulled off Interstate 55 in Channahon Township and shot his entire family inside their Ford sport-utility vehicle.

Vaughn’s wife was shot in the head, and each of his three children were shot twice. Vaughn himself suffered minor gunshot wounds in his wrist and leg.

Days later, Vaughn was arrested at the funeral services and charged with murder.

But now, according to the Chicago Tribune, in a letter to the state Inspector General’s Office, Vaughn’s former private investigator is asking for an outside look into “issues of serious negligence and/or misconduct” by Illinois State Police.

The letter reportedly alleges that state police Sgt. Robert Deel said in a deposition that he had evidence that it was Kimberly Vaughn who actually shot her family, and that she had even fired at her husband’s head – only for the bullet to be deflected by his watch.

However, the letter goes on to say state police quickly dismissed the possibility that anyone but Christopher Vaughn was responsible.

The letter also reportedly explains Kimberly Vaughn was taking medication that could have increased the risk of her having suicidal thoughts.

The Will County State’s Attorney’s office says there is no substance to the report cited by the Tribune.

“We can’t comment on the substance of the report. What I can say is that there are a great many misrepresentations and inaccuracies contained in that report. It advances the defense theory,” said State’s Attorney’s Office spokesman Chuck Pelkie. “The bottom line is this – our prosecutors will present a case when it goes to trial in August, and that’s where we’ll do of our arguing at this point.”

Vaughn’s attorneys were not available for a response.

Authorities have said previously that Vaughn had a magazine article about staging crime scenes and “making the death appear to be a suicide,” which was confiscated by police.

Court records also show Vaughn spent a half-hour at a Plainfield shooting range the night before the shootings, firing at targets with a gun later seized by police from between his dead wife’s feet.

Meanwhile, prosecutors say, an exotic dancer told police Vaughn had written her a poem about “ancient souls” and bad timing, and Vaughn might have explored the possibility of faking his death.

While prosecutors haven’t set forth a motive in the murders, Vaughn’s wife had a $1 million life insurance policy that listed him as a beneficiary. Vaughn also confessed to an affair in December 2006 after a trip to Mexico. The court records show the couple had been fighting, and he would sometimes sleep at the office.

As to the latest letter, Deel, the sergeant who claims there is evidence that Kimberly Vaughn was responsible, is no longer a crime investigator.

The reason is that he was the one who reportedly failed to recover evidence while investigating the death of Drew Peterson’s ex-wife, Kathleen Savio, whose body was found in a dry bathtub in 2004. Her death was initially ruled an accidental drowning, but Peterson has since been charged with her murder. He also awaits trial in Will County.

As a result of Deel’s failure in the Savio case, prosecutors asked that he never process another crime scene again.