SAN LEANDRO (CBS SF) – As students across the nation participated in a national walkout against gun violence, San Leandro High School was placed on lockdown Wednesday after threatening graffiti was found on campus.

San Leandro Police Lt. Ted Henderson told reporters that no threat was found after a systematic room-by-room search of the large high school.

“We searched the school floor-by-floor, looking for anything suspicious,” he said. “We deemed the school safe (after the search).”

Henderson said school security workers found the threatening graffiti on a stall in a boy’s restroom.

“The graffiti was written sometime between the start of school and 9:30 a.m.,” he said.

While some students had taken to social media posting that police had found at least one gun, San Leandro police Lt. Issac Benabou said “there were no guns found.”

He said the postings were the product of rumors spreading through the student body.

Henderson said at this time investigators had no leads on who may have written the graffiti.

The lockdown was lifted at around noon and officials said the school would be closed on Thursday. The graffiti threatened on-campus violence for Thursday.

While the students were not allowed outside to participate in the walkout, some students posed with their banners calling for change and posted pictures on social media.

All other schools in the San Leandro Unified School District are open, police said.