Christopher Vaughn's children pushed back into the seat of a Ford Expedition as their killer stuck a pistol in their faces and shot them one after the other, a ballistics expert testified Monday.

"Each child had about a two- or three-inch-diameter stippling pattern" on their faces, ballistics expert Matthew Lee Noedel said. The size of the powder burns on the faces of the children showed they had been shot from about a foot away, he said.

Noedel was called in by Will County prosecutors in 2010 to figure out the circumstances surrounding the shooting deaths of Vaughn's children—Blake, 8, Cassandra, 11, and Abigayle, 12— and wife, Kimberly.

All four were gunned down in the family's Ford Expedition in June 2007. Christopher Vaughn also took bullets, suffering minor wounds to his left wrist and thigh.

After a passerby found Christopher Vaughn walking along a desolate stretch of the Interstate 55 frontage road in the wake of the killings, he explained to police that it was his wife who shot everyone.

Vaughn was taking his family on an impromptu trip to a Springfield water park. But after making it from his Oswego home to Channahon, he pulled off I-55 onto the frontage road.

That's where Kimberly Vaughn suddenly started shooting everyone, Christopher Vaughn told the police. He was fortunate enough to flee the carnage, he told detectives, and after he got away his wife took her own life by putting a bullet through her chin and into her brain.

Kimblery Vaughn was driven to kill by antidepressants and another medication she was taking for migraine headaches, Vaughn told detectives. She was also supposedly upset over his confession to a sexual affair he claims to have carried on in Mexico.

But Vaughn's story doesn't square with what the ballistic expert told the jury Monday.

For one thing, the bullet wounds to Vaughn's wrist and thigh were on the wrong side of his body. For another, Noedel said, they were inflicted by a gun barrel held flush against his skin.

Also, a pair of shell casings found on the driver's seat likely ended up there after Vaughn got out of the car, he said. And the Taurus semi-automatic pistol all the bullets were fired from cycled completely through after the last round was discharged. Noedel said that indicated the final shot probably wasn't put in Kimberly Vaughn's head by her own hand.

Noedel's testimony will continue Tuesday. Before he took the stand, four other witnesses testified Monday.

The St. Charles, Mo., cop who took Vaughn into custody right before his family's funeral told of arresting the alleged quadruple-murderer. An Aurora lawyer said neither Kimberly nor Christopher Vaughn contacted him about getting a divorce. A legal analyst for an insurance company let the jury know about the $1 million policy Christopher Vaughn was set to collect. And a human resources director from Christopher Vaughn's company showed how he took vacation time for a full week just a month before the killings.

Vaughn—who dressed for his murder trial in cowboy boots, cuffed khaki pants, and open-necked white shirt and brown blazer—spent the week of May 7, 2007, on a "scouting trip" to the Yukon, said Assistant State's Attorney Michael Fitzgerald.

Vaughn planned to move to the Yukon and live in the wilderness after shedding the responsibility of his family by killing them, prosecutors have said.

An email written by Kimberly Vaughn the day before Christopher Vaughn departed for the Yukon showed that she believed he was going on a business trip to his company's Canadian branch office, Fitzgerald said.

Defense attorney Jaya Varghese objected to the jury hearing about Christopher Vaughn allegedly deceiving his wife, as he may have had a "change of plans and she did not put it in the email."

Judge Daniel Rozak allowed the evidence.