Reno, Nev. ? The FBI said Wednesday that 21-year-old college student Luke Helder had confessed to planting 18 pipe bombs in rural mailboxes in five states. Six people were injured in the spree.

Using a map, Helder pointed to towns where he placed eight bombs in Illinois and Iowa on Friday, FBI agent Mark Heavrin said in an affidavit.

Pipe bombing suspect Luke Helder, right, gestures to the media upon returning to Washoe County Jail in Reno, Nev., after a hearing in federal court where he was ordered held without bail for transfer to Iowa. Helder confessed Wednesday to planting bombs in mailboxes. He could be sent to prison for life if convicted.

Helder also described placing other bombs in Nebraska, Colorado and Texas during the weekend and admitted he made 16 more bombs in a room at a Nebraska motel near Omaha, Heavrin said.

The FBI zeroed in on 21-year-old Luke Helder as the suspect in the string of pipe-bomb attacks after his father called police about a disturbing letter in which his son warned, “Mailboxes are exploding,” authorities said Wednesday. The same phrase was in the notes found with the bombs.

Helder was captured Tuesday in Nevada after holding a shotgun to his head during a car chase and telling a friend by telephone “I might have to blow myself away,” authorities said.

The FBI had used the signal from his new cell phone and a tip from a motorist who spotted his Honda Accord to pinpoint his location 1,400 miles from his home in Pine Island, Minn.

During his odyssey halfway across the country, Helder was stopped by police and released three times for traffic violations over the weekend. But that was before his father called police and an all-points-bulletin for Helder was issued on Tuesday.

On Wednesday, the FBI said Helder had confessed to making a total of 24 pipe bombs out of tape, paper clips and Christmas tree bulbs and placing 18 of them in mailboxes in five states, along with anti-government notes. Helder had the six other bombs with him when he was arrested, authorities said. They said the 6-inch bombs were packed with smokeless gunpowder and BBs or nails, and were wrapped in black electrical tape.

Six bombs in all exploded, injuring six people in Iowa and Illinois. The FBI said the final 10 bombs found in Nebraska, Colorado and Texas were not rigged to detonate.

Helder faces federal charges in four states. At a brief hearing in Reno on Wednesday, he was ordered held without bail for transfer to Iowa. He could be sent to prison for life if convicted.

Asked by the judge whether he understood that he does not have to make any statement, Helder replied, “most definitely.” Asked whether he understood that any statements he made can be used against him, he replied “for sure.”

U.S. District Court Magistrate Robert McQuaid Jr. denied a request to release Helder to the custody of his parents.

“It’s apparent to me that he suffers from some apparent mental health problems,” McQuaid said.

The bombs rattled the Midwest and recalled last fall’s anthrax-by-mail attacks as well as the case of the Unabomber, who was turned in his by own brother. The bombings prompted the Postal Service to suspend service in some areas and urge people to leave their mailboxes open so letter carriers could peer inside.

Before Helder’s arrest, criminal profiling experts had speculated that an older man was responsible. But the improbable suspect who emerged proved to be a guitar player in a punk rock band called Apathy and a junior studying art and industrial design at the University of Wisconsin-Stout in Menomonie, Wis.

Helder remained something of an enigma Wednesday. He was described as bright, polite and not given to ranting about politics. In high school, he played football and golf and was in the choir. Until this week, his criminal record showed only a marijuana possession charge last October.

That he could be involved in the case stunned people in his hometown and his college town.

“For heaven’s sake, he’s not a terrorist,” said Rachel Stanton, whose son played in Helder’s band. “Nobody saw this coming.”