A device found at Trident Technical College’s Rivers Avenue campus on Monday contained ammonium nitrate and shrapnel, but Charleston County Sheriff Al Cannon would not say today whether it could have exploded.

Cannon spoke during a noon press conference to address the arrest of William Gregory McGrath in connection with the bomb threat. Jail records show McGrath, 35, was booked into the detention center on Wednesday on charges of unlawful use of a fake bomb and growing marijuana.

He has a bail hearing scheduled for 2 p.m. today.

The school received the threat by email sent to a general account, according to Cannon.

The message led the campus police force to the device, which was wrapped in a plastic bag. It consisted of ammonium nitrate, which is commonly found in fertilizer, as well as an ignition source and some shrapnel, including bits of steel and nails, the sheriff said.

McGrath is a former student of the school, but Cannon said his motive was “convoluted” and remains unknown.

McGrath has previous arrests for driving under the influence and malicious injury to property, but dispositions of those cases were not immediately available, according to State Law Enforcement Division records.

Authorities said that they think he acted alone and that no other schools were in danger.

The emails led investigators, including local authorities and federal terrorism experts from several agencies, to a residence associated with McGrath.

They searched the home and a nearby storage shed and found materials consistent with the device, along with growing marijuana plants.

The FBI, Bureau of Alcohol Tobacco Firearms and Explosives, Homeland Security Investigations, North Charleston Police Department, and Trident Technical College assisted in the joint investigations.

Police blocked off the school’s Rivers Avenue campus for several hours Monday while the squad investigated the bomb threat.

An emergency text message sent at 4:44 p.m. warned students to stay away from the campus, and said that the school’s student center, building 410, would be evacuated.

The campus is on winter break, and not many people were there when the threat was made, school spokesman David Hansen said.

The bomb squad brought in a robot to collect the bag. They later determined the item was safe.