Witnesses said the Chevrolet crossed the double-yellow line on 12/121 shortly before the wreck, according to Paulson, though the reason remained unclear late Sunday. The impact destroyed the noses of both SUVs, their sheet metal crumpled almost to their front windshields.

“Both vehicles were at or near the speed limit — there was almost no braking except maybe a last-second swerve,” he said. “They hit almost front license plate to front license plate; it was about as head-on as you can get. They may have tried to swerve but there was no time to get out of the way.”

The boy was in a booster seat and the surviving occupants all were using their seat belts, according to Paulson, who said there was no early indication of alcohol or drug use by either driver.

The crash occurred on a stretch of highway just west of the di Rosa art gallery and the Domaine Carneros Winery.

Authorities closed 12/121 in both directions for more than three hours after the crash, detouring drivers onto Duhig Road. The highway reopened at 6:45 p.m.