A 21-year-old university student from the midwestern United States has confessed to planting a series of pipe bombs in rural mailboxes in five states. Luke Helder was arrested Tuesday evening in Nevada, four days after the first of his mailbox bombs exploded.

The FBI says Luke Helder used a map to point to towns in Illinois and Iowa where he planted pipe bombs that were found Friday. Several of those bombs exploded when the mailboxes he placed them in were opened. Six people were hurt. Mr. Helder also described placing bombs in additional mailboxes in the states of Nebraska, Colorado and Texas.

Mr. Helder had six more pipe bombs in his car when police stopped him Tuesday afternoon after a 60-kilometer high-speed chase.

Authorities began looking for Mr. Helder after his father called police late Monday about a letter from his son that contained references to the bombings.

A motorist near Reno, Nevada, who had heard Mr. Helder's car description and license plate number in a news report Tuesday spotted the car and called police.

Federal prosecutors are charging Mr. Helder with using an explosive to maliciously destroy property affecting interstate commerce and with using a destructive device to commit a crime of violence. The charges carry penalties of up to life in prison and fines of $250,000.

Mr. Helder was a student at the University of Wisconsin-Stout in the town of Menomonie. His classmate Andrew Armstrong is among those on campus surprised their friend could be involved in something like this.

"The type of kid he was, I didn't even see him knowing how to do something like that," he said. "That part kind of astonishes me."

The 18 mailbox bombs found since Friday were all accompanied by anti-government notes. Mr. Helder sent similar notes to a university newspaper in Wisconsin, and to his father in Minnesota.

Officials say Mr. Helder was stopped by police three times on Saturday and Sunday in the states of Nebraska, Oklahoma and Colorado: twice for speeding and once for not wearing a seat belt. He was given citations and allowed to go because none of the officers knew at that time what Mr. Helder was up to.