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 numerous replies appeared on the site 4chan.org hours before a 20-year-old gunman opened fire at Umpqua Community College in Roseburg, Oregon, on Thursday, killing and wounding an as yet undetermined number of people.
Reuters viewed the thread but could not verify its authenticity or whether it had any connection to Thursday's shooting. Emailed questions to federal law enforcement officials about the postings were not immediately answered.
A comparable posting three years ago ahead of the school massacre in Newtown, Connecticut, was later discredited by police.
"Some of you guys are alright," the thread began on Wednesday evening. "Don't go to school tomorrow if you are in the northwest."
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 continued: "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, and some of them egged him on, advising him on tactics and weapons, in a mildly sarcastic tone.
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.
In that case, according to the Connecticut State Police final report on the attack, an unknown individual posted two days beforehand: "I'm going to kill myself on Friday and it will make the news. be watching at 9:00 am."
Another anonymous individual asked "Where at?" to which the first individual responded: "I live in Connecticut, that’s as much as I'll say."
Police later determined the posting had no relation to the Newtown shooting, which was carried out by a 20-year-old gunman, Adam Lanza.
4chan, launched in 2003, was originally popular mostly among devotees of Japanese anime comic book art, but has since become associated with Internet subculture. It has 180,784 users. Registration is not required, nor it is possible, except for staff, according to its website.
(Reporting by Angela Moon; Additional reporting by Fiona Ortiz in Chicago and Alex Dobuzinskis in Los Angeles, Saqib Iqbal Ahmed in New York; Editing by Dan Burns, Toni Reinhold)