NEW YORK • A chain of messages posted to an anonymous chat board known for its explicit conversation threads and graphic images included an ominous warning on Wednesday evening not to go to school on Thursday "if you are in the northwest".

The anonymous postings and many replies appeared on the site 4chan.org hours before a 20-year-old gunman opened fire at Umpqua Community College in Roseburg, Oregon.

"Some of you guys are alright," the thread began on Wednesday evening. "Don't go to school tomorrow if you are in the north-west."

The posting, since removed from the site, continued: "Happening thread will be posted tomorrow morning so long space robots." About four minutes later, the same poster said: "Will post again in am, 10 min countdown. Won't say more to much to prepare."

Other posters in the thread assumed that the original poster was going to shoot people at a school. Some of them even egged him on, advising him on tactics and weapons, in a mildly sarcastic tone.

Reuters viewed the thread but could not verify its authenticity or whether it had any connection to the shooting. E-mailed questions to federal law enforcement officials about the postings were not immediately answered.

CNN and other news outlets reported that authorities were investigating possible threats made on social media in advance of the attack.

It is not uncommon for purported threats to circulate on social media forums in the aftermath of mass shootings.

Authorities investigated a posting made on 4chan in advance of the December 2012 massacre at Sandy Hook Elementary School in Newtown that left 20 first-graders and six educators dead. Police later determined the posting had no relation to the shooting, which was carried out by Adam Lanza, 20.

REUTERS