The Virginia Tech campus was under lockdown Thursday morning, after an alert posted on the official campus Web site warned that a gunman had been sighted in campus -- eliciting memories of the 2007 shooting that left 33 dead.

According to the alert posted at 9:09 a.m., a white male, "6 feet tall, with light brown hair outside of New Residence Hall East" was seen holding what may have been a handgun, which was covered "by a cloth or covering of some sort."

However, when the security officials immediately arrived at the scene, no gunman matching the description was found. Officers, which come from at least five law enforcement agencies such as the Virginia State Police, the Blacksburg Police Department and Montgomery County Sheriff's Office, kept patrolling the campus, "continuing to look for the subject."

The alert urged the people on campus to "Stay inside. Secure doors."

"People on campus stay indoors until further notice," the university ordered.

Larry Hincker, associate vice president for university relations, said later at a news conference that the lockdown remained in effect as a precaution.

Summer classes are currently in session at the campus. Hincker estimated that about several thousand students are on the grounds.

When the alert was announced, some students were in class. They have locked themselves inside their classrooms as a precaution.

"I go to VT and everyone is locked down in classrooms as of right now ... I have faith that our officers will take care of it as best as they can," one student tweeted.

"At this point we have not been able to confirm that there is gun on campus," Lt. Steve Taylor, a spokesman for the Blacksburg Police Department said.

In 2007, Seung-Hui Cho, a 23-year-old Korean-American Virginia Tech student who was majoring in English, shot dead 32 students and faculty members and wounded 25 others and then committed suicide by shooting himself.

While Cho studied in Virginia Tech, he had alarmed professors with his writings and was ordered to see a university psychologist.

The case shocked the nation and was ranked as the worst shooting incident in U.S. history.

READ: Virginia Tech Gunman: Authorities Release Composite Sketch of Suspect

READ: Virginia Tech Lockdown: Remembering the 2007 Victims