A Twin Cities couple’s decision to scrawl their junior high-like profession of love into a sandstone pillar in Montana could become a national scandal.

Their names now stand etched into Pompeys Pillar, a national monument bearing the 207-year-old signature of one of this nation’s most revered explorers, Capt. William Clark of Lewis and Clark fame.

Clark’s signature, carved in the stone on July 25, 1806, is roughly three feet away from the new inscription, which includes the Plymouth couple’s first names with a “+” in the middle, a heart and the date “10/10/13.”

The discovery of the new carving, made while the site was officially closed during the government shutdown, has prompted a federal investigation that could result in a felony charge against the couple. They are not being named because they have so far not been charged with a crime.

For some Montana residents who live near the Pillar, about 25 miles east of Billings, the law can’t move quick enough.

“A lot of people are heartbroken about the vandalism because of the historical value and the meaning of Pompeys Pillar,” said Jonathan Peart, executive director of the nonprofit Friends of Pompeys Pillar.

In Huntley, Mont., 16 miles from the monument, heartbreak has escalated into anger. Locals at Stroker’s Tavern want to see the couple returned to Yellowstone County.

The pair deserve more “than just a slap on the hand,” said Gina Loose, a 51-year-old construction company manager. “They can come do the cleaning. They can come do the restoration. They can do it during the middle of summer when the rattlesnakes are out. See how they like it.”

Scott Eggemeyer, a 46-year-old customer at Stroker’s, put down his ribs to fume.

“This is what’s wrong with our country today, is these kids don’t know a damn thing about what made our country great,’’ Eggemeyer said. “They don’t even know who Meriwether Lewis was.”

Kristen Lenhardt, a spokeswoman for the federal Bureau of Land Management in Billings, said that Clark left his signature some three weeks after splitting up with Lewis on their legendary journey west. “He stopped and signed the pillar. It’s the last physical remaining evidence of Clark’s journey,” which began in 1803.

The man accused of the vandalism didn’t return a phone message or e-mail Friday.

The couple are in their 20s and fledgling musicians. They entered the park during the recent government shutdown and were seen by a Yellowstone County sheriff’s deputy. The deputy warned them that they were trespassing before taking their names and sending them on their way without citing them, said Sheriff’s Capt. Bill Michaelis.

Authorities later reviewed video surveillance that showed the two had shrugged off the deputy and warning signs, jumped a rail and “carved their names in the rock,” Michaelis said.