CASTRO VALLEY — It had all the signs of an explosive device, authorities said.

A ring holding wires that ran to a cooler like box that was emitting a humming sound. There was also the word “dangerous” on a note on the packaging.

It was not until after the Alameda County Sheriff’s Bomb Squad blew it up Monday evening that it was determined not to be a bomb, but equipment used to measure the ozone layer that had been carried by a balloon.

The styrofoam box containing the equipment known as an ozonesonde and remains of its parachute landed about 5:20 p.m. Monday on Redwood Road near Grove Way. People who saw it called the California Highway Patrol. which after taking a look decided to call the bomb squad.

Sheriff’s Sgt. Ray Kelly said bomb squad members “saw components similar to something you might see in a real bomb, and felt it was in everyone’s best interests to detonate it.”

Some people in the area were told to evacuate while others were told to shelter in place.

After the bomb squad deputies detonated the box, they learned it contained measuring instruments for the ozone layer that had been launched by San Jose State University students.

Deputies also found more of the note on university letterhead where the word “not” had been separated from dangerous.

Kelly said based on what they observed, the bomb squad had no choice but to detonate the device, and he thanked the community for notifying authorities. He said there is particular concern now “about drones or other airborne devices dropping bombs. Thankfully it turned out not to be real,” Kelly said.

Professor Sen Chaio, director of the Center for Applied Atmospheric Research and Education at San Jose State University, said the balloon and equipment was launched by students probably from Half Moon Bay and the box contained equipment to send information to ground receivers.

He said the parachute is supposed to make sure the equipment, worth about $1,000, lands without damage after the plastic balloon carrying it bursts in the atmosphere. The balloons can go as high as 25 kilometers, he said. People who find the equipment receive rewards for returning it and it can be reused.

Chiao said it is the first time anything like this has happened since they began doing upper air ozone measurement in 2016 and that the flights will probably continue because of their scientific value. “It’s an effort for better monitoring our environment and air quality,” he said.

Kelly said it was a “learning experience” for everyone involved.

“We are now more aware that this kind of equipment is flying around the atmosphere and they are more aware they need to better label it as a piece of scientific equipment.”