According to deputies, a man wearing a fluorescent orange mask and a blue jacket walked into the bank and demanded money from two tellers. The employees put money into a backpack and the man ran out of the bank, Investigator Pete Lazich said.
As he ran, a man followed him outside, Lazich said. The man told investigators that he thought he saw the robber throw something into a silver Ford Escort that was parked outside as he ran by.
The customer and his brother got into their car and followed the Escort through Williamsport, copying down the make, model and tag number.
He gave the information to police along with descriptions of the robber and the driver and the direction in which the vehicle was headed.
"I would say that's a pretty good witness," Lazich said.
Based on the description, deputies arrested Jacob Knapp a short time later.
Lazich said he and Investigator Chris Weaver were on their way into Williamsport in response to the bank robbery report when they saw a car matching the description stopped at a red light on Md. 63.
The investigators turned around and pursued the car, stopping it on Md. 68 near the Conococheague Creek, Lazich said. The driver, Jacob Knapp, was arrested.
Meanwhile, the robber had run into an alley about a half-block from the bank. Deputy Kenneth Cain and Trooper 1st Class D.A. Harper and Sgt. Steve McCarty arrested David Knapp near the Williamsport American Legion, authorities said.
Officers found a large amount of money in his pockets, Lazich said.
Sheriff's deputies later recovered the jacket, the mask, the backpack and a ratchet in the alley, Lazich said.
Sgt. Randy Wilkinson said authorities initially thought the robber had a gun but none was found.
Lazich said the customer's daring and quick thinking dramatically improved the chances of capturing the robbers, although he added that witnesses should not take unnecessary risks.
"Usually, we don't have a description, a tag number or a direction of travel because people stay in the bank like they're supposed to," he said.