SALIDA, Colo. -- A nationwide string of mailbox pipe bomb threats spread to Colorado Monday with the discovery of pipe bombs in Salida and in Pueblo County and authorities say there could be more.

The mailbox pipe bombs found in the country so far have been found in groups so officials fear more may be found in Colorado.

The first eight bombs were found in Illinios and Iowa, somewhat clustered together, 7NEWS reported. In Nebraska, eight others were found, and again, they were in a cluster.

Then one was found in Salida, and one in Armarillo, Tex.

Based on the previous pattern, authorities say there could be more bombs within a 100 mile radius of Salida.

The latest discovery in Colorado was made about 9 p.m. Monday in a field near Interstate 25 in rural Pueblo County. According to the local sheriff's department, the second bomb apparently doesn't fit the characteristics of other devices found.

However, the bomb in Salida appears to be linked to 17 others left in mailboxes across the Midwest.

Federal investigators and police were called to Salida shortly after 1 p.m., when a man found a pipe bomb in his mailbox.

The area, about 12 blocks from downtown, was cordoned off until a bomb squad unit could inspect the mailbox.

MAILBOX EXPLOSIVES Locations Of Pipe Bombs Timeline Of Mailbox Explosives Text Of Letter Found Near Bombs The U.S. Postal Service offers these indicators of a suspicious package: From someone you don't know No return address, or one that looks fake Addressed by hand Lumpy appearance Sealed excessively with tape Marked with restrictions like "personal" or "confidential"

Postal Service spokesman Al DeSarro described the device as a metal pipe with two wires protruding from it. It was in a plastic bag and contained a folded piece of paper.

The Chaffee County Sheriff's Office said that the pipe was 6 inches long and three-fourths of an inch in diameter.

"We've had no advance warning about this," said Ann Antanasio, Denver FBI agent. "The investigation is in its initial stages at this point."

"Initial description of the device indicates that its construction is consistent with those found recently in Nebraska and Iowa," said a press release from the Denver Federal Bureau of Investigation.

Like other bombs found in the other states, this bomb contained an anti-government letter.

Although officials have not released what the Colorado letter says, the others say, "If the government controls what you want to do, they control what you can do."

It says, "I'm obtaining your attention the only way I can" and warns, "more attention getters are on the way…"

Neither bomb exploded or injured anyone.

The Coloradto postmaster has instructed all Colorado letter carriers not to deliver mail to any closed mailbox.

FBI officials believe that the person who is leaving these bombs is moving westward.

Pipe Bomb Found In Texas

On Tuesday, the FBI also confirmed that a pipe bomb found at Amarillo, Tex., also appears to be from the same source as 17 others.

The Texas Department of Public Safety and the FBI are looking for a 21-year-old man in connection with the Texas mailbox bombs.

Texas DPS officials say Luke John Helder -- described as a 150-pound, 5 feet 9-inch tall white man with brown hair and green eyes -- reportedly is driving a gray, 1992 four-door Honda Accord with Minnesota license plate number EZL-873.

Officials at the University of Wisconsin-Stout in Menomonie, Wis., say Helder is a student at the university.

Helder is from Pine Island, Minn., a town of about 2,000 people about 17 miles north of Rochester, Minn. Rochester is about from 250 miles from Eldridge, Iowa, where the first pipe bomb was found, on Friday.

Bombs Across Country

The explosive devices were found Friday, Saturday and Sunday in Nebraska, Iowa and Illinois.

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New Rules Affect City Residents, Too Letter Carriers Take Extra Precautions Carriers Say Delivery Will Continue

Nobody has been hurt in Nebraska. However, another eight bombs found Friday in east Iowa and west Illinois injured six people -- four postal workers and two residents.

The mailbox bombings in the Midwest are probably the work of one person, according to the FBI special agent in charge of the case.

While not ruling out a group in the bombings, officials said that each device has been accompanied by a letter written in the first person. Investigators believe the bomber tried to communicate with the government in the past and has "unresolved issues," 7NEWS reported.

Investigators found those typed notes with some of the bombs in Iowa and Illinois. Those letters were filled with anti-government remarks. A note attached to one of the Nebraska bombs was severely damaged when the bomb was destroyed, so it is not yet known what the letter said.

The Nebraska State Patrol investigated an eighth suspected pipe bomb, but it turned out to be a copycat prank. An 18-year-old from St. Paul, Neb., was arrested for putting the toy in a mailbox.

So far, all of the mailbox pipe bombs have been placed in rural areas. However, the postal service is temporarily changing its delivery rules for all customers in rural and urban areas until a suspect is identified or arrested.

First, all mailboxes by the side of the road must be left open, or the door must be removed. Mail will not be delivered to any curbside mailbox that is closed.

Second, the postal service asks that residents put their name and address on the inside of the mailbox, or the door if it is still attached.

The new rules do not apply to boxes attached to homes or porches. A U.S. Postal inspector said the people leaving the bombs have not approached homes because they do not want to draw attention to themselves.

If you do not receive your mail, contact your local post office.

Letter carriers will still pick up mail, but only if it is left in an open mailbox. Postal administrators said they would prefer that customers bring their mail to their local post office, or hand deliver it to a letter carrier.

Anyone with information about the bombs should call the FBI at (866) TIP-2-FBI, which is (866) 847-2324.