The Denver Public Library closed all locations on Wednesday after it received what it called an unspecified threat overnight, Denver Gazette news partner KUSA reported.

The library system was working with law enforcement to decide when it’s safe to reopen. Bookmobile stops were also canceled.

Denver Public Library, which has 27 branches across the city, provides community services and public access to computers and technology, in addition to books and other media.

Read the full story from KUSA here.