The explosion at the library, located at 400 South and 200 East, caused no injuries but forced the evacuation of about 400 patrons.
Saying the explosion was not accidental, Merino called the bomb an "improvised device." He declined to say how it was constructed, what explosive it used or how it was detonated.
The explosion occurred about 2:30 p.m. Friday in a lounge area on the 3rd floor's northwest corner. Some patrons described the detonation as sounding like a gunshot and said they could smell gunpowder or natural gas afterward.
A softball-size hole was blown in one window. Merino said a combination of blast force and debris from the bomb probably caused the hole.
The library has cameras, but none was capturing that area of the library. Merino said other video footage from the library is being reviewed.
Library business had returned to normal by Saturday. Library Director Nancy Tessman said no new security measures are in place.
The police department, the FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives are investigating.
Anyone with information about the bombing can call the police department's tip hot line at 801-799-INFO.