Idaho Dad Charged with Manslaughter for Car Crash That Killed Son

Police say an Idaho dad was on his phone behind the wheel ... moments before the car crash that killed his young son.

Austin Summers was charged with manslaughter last week for the death of his 7-year-old son Graham, who tragically died when he rear-ended another car in November, according to the Idaho Statesman.

According to the outlet, the affidavit says Austin, whose blood tested positive for cocaine and opiates following the crash, was flying down Interstate 84 in his GMC pickup truck at about 75 mph when he slammed into a Ford F450 that was in the far right lane with its hazards on, crawling at about 7 mph because of a mechanical issue.

The court doc says Graham was found dead in the passenger seat and hadn't been wearing a seatbelt. Austin's 2 other kids -- 3 and 5 years old -- were injured in the backseat ... and were taken to the hospital.

According to the outlet, investigators got a search warrant for Austin's phone, which showed he sent and received over a dozen messages, scrolled Instagram, was using Amazon, and accessing photos between 2:45 p.m. and 3:02 p.m. ... the time of the crash.

The most bone-chilling part might be that they found he'd taken a photo and video of a different crash right before his own ... and allegedly drafted an IG story with a cut-off message that said, "Avoid th[e] freeway heading ..."

Graham's mother, Ashley Summers -- who's separated from Austin -- gave cops a court doc where Austin admitted to using cocaine the Saturday before the crash, and another one where he said he was aware Graham's seatbelt had been unbuckled.

He reportedly wrote in the doc ... "While I am devastated by the loss of my son, I take responsibility not in that I was impaired by any means, or that Graham unilaterally took his seat belt off while I was driving on the freeway, but that I should have immediately pulled over and made sure he was properly buckled. That will haunt me forever."

Ashley also gave cops photos and videos that show her ex in cars with their kids, where they're not buckled up or in car seats ... the affidavit says.

Court records show Austin is facing one felony charge of vehicular manslaughter and two charges of injury to a child. He's being held at Ada County Jail on a $500K bond ... with a bond reduction hearing scheduled for August 6.

Graham was 7 years old.