 Montgomery County Fire Rescue spokesperson Pete Perringer says it appears that a vehicle collided with the tractor-trailer then left the scene just before 6.30 this morning. He says the big rig caught fire after its saddle tank full of fuel ruptured. About 15 to 20 barrels of crushed tungsten metal spilled onto I-495 near the I-270 spur, spreading debris into the roadway and causing slick road conditions. Traffic had to be diverted around the area for about four hours.