State police and Boston police on motorcycles pushed the crowd first back across the street and then evacuated everyone from the area around the hotel skyscraper.

Officials also evacuated the nearby Copley Place Mall and closed the Boston Public Library and the Prudential Center—which had already posted extra security at its entrances in the wake of Tuesday’s attacks.

At the base of the hotel, hundreds of onlookers waited anxiously at the yellow police lines.

“I keep thinking that at any moment a bomb’s going to come out the side of the building,” onlooker Mark S. Elsmore said, as he looked up at the 36-story glass-shrouded hotel.

Tuesday’s horrific tragedy appeared to have shaken everyone at the scene in Back Bay, from onlookers to emergency personnel.

Boston resident Rubin Toledo and his son, Pablo, showed up at the hotel about an hour into the incident. They stood quietly at the edge of the crowd, waving American flags held high over their heads.

Toledo said Tuesday’s attacks had claimed the life of one of his cousins, Sonia Morales Puopolo, who had been aboard hijacked American Airlines Flight 11, one of the flights that crashed into the World Trade Center. He said that he hoped any suspects detained yesterday would face swift punishment.