Porterville Fire officials identified the firefighter killed while fighting the blaze as Porterville Fire Capt. Raymond Figueroa. The 35-year-old started his career with the department in 2007.
Authorities say the body of 25-year-old Patrick Jones, a Porterville firefighter, was located late Wednesday night.
Officials say two teenagers were seen running from the building as the flames grew. Porterville police detectives tracked down the teens and arrested them. They have been booked into the Tulare County jail on manslaughter charges.
WATCH: Authorities provide updates on deadly Porterville library fire
More than 50 firefighters from Cal Fire, Tulare County and Fresno County helped Porterville city fire crews control the deadly fire.
"I'm not familiar with the building, but a library of this size and magnitude, there's a significant fuel load in that building," says Tulare Co. Fire Chief Charlie Norman.
Scanner traffic of firefighters battling Porterville library fire
ABC30 insiders captured the billowing smoke and flames as they called friends and loved ones who may have been inside when it started on Tuesday afternoon.
"I called a couple of people I knew in there to make sure they were okay," says Cecelia Nunes Martinez. It's just so sad that this is happening in Porterville.
Fire officials say the library was built in the early '50s and it did not have a sprinkler system.
Procession held for Porterville firefighter killed
"This has taxed a lot of our resources, and a lot of our officers and our investigating units are out there," says Porterville Police Chief Eric Kroutil.
Porterville residents watched on as the beloved building burned to the ground, keeping the firefighters in their thoughts.
Martinez also hopes the city will rebuild and that this won't be the final chapter for the Porterville library.
Porterville City Hall will remained closed Wednesday as the investigation continues.
Valley agencies and officials have begun to express their grief in honor of the firefighter and the Porterville Fire Department.