A getaway driver will receive institutional drug treatment for her role in a near-fatal robbery.

Letah Gerdes, 40, of Atchison, Kan., pleaded guilty earlier to hindering prosecution for what she thought was a harmless beer run last June. The prosecution agreed that Ms. Gerdes didn’t know the true intention of the trip beforehand, but afterward she knew a crime had been committed.

She drove Sean Cave on his return trip to a Winthrop, Mo., convenience store. Mr. Cave wanted to collect video surveillance equipment from his earlier robbery of the store, which included him slashing the clerk’s throat.

But on entering the store the second time, Mr. Cave found the clerk, Kim Dejonge, still alive. He again slashed her throat, took a computer tower, and had Ms. Gerdes drive him back across the river.

Ms. Gerdes testified earlier that she knew something was wrong as the pair headed over the bridge back to Atchison. Ms. Gerdes dropped Mr. Cave off in Atchison, then drove back into Missouri to visit a friend.

At that point, she passed the scene of the crime, and a witness recognized the car, said Assistant Buchanan County Prosecutor Ron Holliday. Once authorities found Ms. Gerdes, she cooperated.

“Obviously this was one of the most horrendous crimes that has occurred in Buchanan County over the last five years,” Mr. Holliday said. “... The defendant’s role, although limited, was instrumental in aiding Sean Cave.”

Mr. Cave is serving life in prison, plus 50 years.

On Wednesday, Buchanan County Circuit Judge Pat Robb sentenced Ms. Gerdes to a 120-day institutional drug treatment program, which had been the prosecution’s recommendation. She would have to serve up to four years in prison if she fails to complete the program and probation.

Mr. Holliday noted that Ms. Gerdes had been smoking crack the day of the robbery.

Ms. Gerdes’ attorney, public defender Michelle Davidson, said Ms. Gerdes is in obvious need of drug treatment, and that she is deemed disabled due to a mental health diagnosis. Mr. Robb agreed that Ms. Gerdes made a choice to drive Mr. Cave away from the scene, but he agreed that she would benefit most from treatment, due to her long history of drug abuse.

Joe Blumberg can be reached at joeblumberg@npgco.com.