GORHAM – Using a high-pressure stream of water, a robot detonated a homemade bomb that was placed in a rural mailbox.

But police were still trying to find out how the plastic soda bottle filled with chemicals got there. A similar device exploded in an adjacent rural mailbox Tuesday night, Gorham Police Chief Ronald Shepherd said.

Gorham police and the State Fire Marshal’s Office are investigating the case, which surfaced just days after the University of Southern Maine evacuated its campuses in Lewiston, Gorham and Portland following bomb threats.

There was no indication that the mailbox bombs had any relation to the earlier threats, Shepherd said.

Susan Neily of Gorham, who called police to report that her mailbox had been destroyed, said she heard a loud bang at about 10:30 p.m. Tuesday but thought it was a truck’s tailgate slamming.

A police officer who went to investigate found a suspected explosive device in a mailbox attached to the same pole. Police barred traffic from the rural road while the Portland bomb squad arrived.

On Wednesday morning, the Portland Police Department’s 4-foot-tall robot, equipped with lights, camera and a mechanical arm, disabled the bomb. With officers operating the robot from 350 feet away, it fired a high-pressure stream of water that broke open the soda bottle.

“Essentially, it tears it apart so it won’t detonate,” said Portland Police Capt. Russell Gauvin. “It’s the only way to neutralize it.” Liquid that spilled out was collected and sent to the Maine State Police Crime Lab in Augusta.

Portland’s robot was purchased last year with a grant from the Department of Homeland Security. The Maine State Police and the Bangor Police Department also have robots.