A six-hour lockdown at the University of Toronto over a report of a gunman at the St. George downtown campus ended Monday without a suspect and tested the university's readiness for a school shooting after the high-profile massacre in Orlando.

Dozens of tactical officers and police dogs swept through three faculty of law buildings just north of Queen's Park after police received two calls just after 9 a.m. about a masked man dressed in black and carrying a gun. When no suspect was found, police lifted the lockdown at 3:20 p.m.

"We're satisfied at this time that there's no person with a gun," Sergeant Sean Cassidy said. "I'm sure the investigators will continue to speak to the original complainant. But at this time, we don't have concerns over public safety."

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Police said the first call came from campus security at 9:20 a.m. about an unidentified man wearing all black and a surgical mask at U of T's Trinity College, which was under construction and closed to the public. Twenty minutes later, U of T community police received a call about a possible gun on campus.

The caller "suspected that this person had a gun, but nobody actually saw a gun on the person," Sgt. Cassidy said.

Tap for more info on key locations on the St. George campus mentioned in this story.

U of T campus police relayed the report to Toronto Police, who were unable to reach the caller until the afternoon. Although the link between the two calls was unclear, members of the Toronto Police's Emergency Task Force quickly swarmed the campus.

The university issued its first campus-wide alert over e-mail and text at 10:13 a.m., announcing lockdowns in eight buildings and surrounding streets. Queen's Park and nearby hospitals, including the Hospital for Sick Children and Women's College Hospital, were placed under lockdown. Museum station, a subway stop close to campus on the Yonge-University-Spadina line, was also shut down in response to the police search, theTTC said.

The lockdown came a day after a mass shooting at a gay nightclub in Orlando, Fla. – the worst in recent U.S. history. "I thought of Orlando immediately," said Sara Vicinaiz, a student from Texas who was among 50 international students visiting the faculty of music for the two-week Tafelmusik Baroque Summer Institute.

"I didn't notice a sense of fear. But people were a little on edge, especially after Orlando," said Charlotte Nediger, a music instructor with the institute.

Shortly after the music lecture started at 9 a.m., students said two police officers escorted a student into the third-floor classroom of the Edward Johnson Building and ordered them to all stay in the room.

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Sarah Bates, another student from Texas, said the instructor kept teaching while nervous classmates quickly started checking their phones for news.

Ms. Bates loaded the U of T website and said she found it strange that no alert had been posted, despite police officers with guns drawn visible from the windows. "It took them so long to clarify that there was a situation. That bothered me," Ms. Bates said. "In the U.S., if there might be a situation, we're getting e-mails and texts."

David Estok, U of T's vice-president of communications, said the university responded "almost immediately" with a banner alert on the U of T's homepage and campus-wide alerts over e-mail and text message. The U of T sent four more alerts throughout the day to students, faculty and staff.

"I'm not aware of any complaints and I'll certainly look into it to see if there are any. From our perspective, we thought it was very smoothly handled," Mr. Estok said.

Mr. Estok said he expects the university will debrief with Toronto police to determine how to improve future responses.

As the day progressed, dozens of onlookers gathered behind police tape at the south side of Queen's Park to observe the heavy police presence. At about noon police escorted a stream of students and staff out of the buildings and on to Toronto transit buses for identification.

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Nearby, students clad in suits and dresses posed for photos to ring in convocation ceremonies scheduled for the Rotman School of Management and St. Michael's College. U of T president Meric Gertler attended the ceremonies and later issued a statement thanking the police for "their fast action and for the care they took to ensure the safety of everyone in the area."

"I think I speak for many of us when I say that this has been a distressing day, but I am very relieved at the outcome," he said.

With a report from Laurent Bastien