Scottsbluff, Neb. (ap) — The annual revival of the Pony Express has been chosen as the official event to celebrate Nebraska’s 150th birthday.

More than 700 riders began the mail service trip by horse in Missouri on Monday, the Scottsbluff Star-Herald reported. They’ll travel nearly 2,000 miles via the Pony Express National Historic Trail through Missouri, Kansas, Nebraska, Colorado Wyoming, Utah, Nevada and California by June 15.

Riders were expected to enter Nebraska on Tuesday. The 565-mile route in Nebraska is the longest trek among the eight states.

This year’s commemorative letter honors Nebraska’s sesquicentennial, describing the Great Platte River Road’s history and featuring the iconic Chimney Rock. The envelope will also showcase the U.S. Postal Service-issued Nebraska sesquicentennial postage stamp.

Max Cawiezel has been the trail captain of the local ride for 25 years and is in charge of the far western leg of the ride. He recruits and organizes local riders, gives their applications to the state organization and decides the order they’ll ride in.

“On the day of the ride, I give them their oath,” he said. The oath required original riders not use profane language, drink or fight.

Riders will carry the mail in a leather pouch, or mochila.

“Each rider gets to sign the mochila and receives a Bible,” Cawiezel said. “The day of the ride they get it stamped with the year and the anniversary of the ride. This year is special because of the 150th anniversary of Nebraska. There’s going to be a special stamp for that.”

The mochila will also carry a GPS device to track the mail and issue progress reports. Cawiezel said people can follow the trail on their computers.

When the original Pony Express operated for 19 months in the 1860s, it was the most direct means of connecting the new state of California with the rest of the U.S. During its operation, the Pony Express delivered about 35,000 letters.