Police say a vacant school library in Ashington "will need to be pulled down" after it went up in flames causing an £100,000 in damage.
Emergency crews were called to Josephine Butler Academy on Ashington’s Academy Road at 2pm on July 10.
The fire is being treated as malicious and police say this is the third fire in as many days.
Police say they are conducting a number of enquiries and reviewing CCTV.
Anyone with any information is asked to contact police using reference number 557 10/07/20 or by emailing the leading officer on email@example.com.
Alternatively you can report via he independent charity Crimestoppers anonymously on 0800 555 111.