The first thought that crossed Amy Holder's mind when she was shot in the head wasn't the pain, the blood that ran down her face or the broken glass that sprayed across her lap.

The Paris Crossing, Ind., truck driver's first priority was keeping her 78,500-pound semi from swerving into other traffic or the ditch.

"I don't think I initially thought I was bleeding," said Holder, 34, who hauls kitchen appliances and other freight from Chicago to Dallas with her husband. "I didn't want to wreck the truck. I just did what I had to do."

The small-caliber bullet shattered the driver-side window last week as she cruised I-44 about 70 miles southwest of St. Louis. She suffered no serious head injuries and was released from a hospital the next day with a few stitches.

Holder is the ninth driver since February whose window has exploded on that same stretch of road near Bourbon, Mo. Investigators want to know if the incidents are related.

Already, the case has alarmed investigators and drawn nationwide attention just seven months after a suspect was arrested in connection with more than 20 sniper shootings on I-270 near Columbus, Ohio. One woman died in that shooting spree.

The bullet fragments in Holder's head and the truck's cab are the first sign of gunfire among the nine Missouri reports.

"We have nothing proofwise to show the others were definitely bullets," said Chris Crawford, an assistant chief of communications for the Missouri State Highway Patrol. "It could have been a rock; it could have been some type of debris. There could have been many situations. We do not have the evidence to say it was a sniper."

Investigators believe, however, that Holder's shooter pulled the trigger from a fixed position.