NATICK -- A split-second slower or faster and everything would have been different for Eric Brack.

Instead, as Brack, an 18-year-old runner and a recent graduate of Holliston High School, was driving west on Route 135 in Natick on Tuesday with his father, an underground explosion heaved a 100-pound manhole cover 25 feet in the air and through the windshield, leaving him in critical condition yesterday.

''How do you figure the timing on that?" said Natick police Lieutenant Nick Mabardy, a spokesman for the department. ''It's so scary, really. You drive down the street and you watch for other cars, you watch for kids on bikes, you watch for runners. How do you figure this is going to occur?"

Brack and his father, Karl, were heading west on Route 135 near Lake Cochituate at 7:20 p.m. when the explosion occurred directly in front of their Chevy Tahoe.

Police said a witness fishing nearby reported that flames flew 25 feet in the air after the blast, which was believed to have been responsible for a power outage that darkened 3,000 Natick homes and businesses for about an hour, according to NStar officials.

A crime scene technician with Natick police, Detective James M. Ordway, said the manhole cover never touched the hood or roof of the vehicle. After entering the SUV, it snapped back the front passenger seat, broke the rear seat, and landed in the vehicle's cargo area.

Calling the accident ''the freakiest thing I have ever seen in my career," Ordway was still shocked by the event yesterday. ''You cannot duplicate this; I'll bet my retirement pension," he said.

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