A teenage thrill-chaser slipped through a fence, eluded a security guard and climbed to the top of 1 World Trade Center, authorities said Thursday amid concerns about a seemingly audacious breach at what is supposed to be one of the world's most secure sites.

Justin Casquejo, a 16-year-old described by a friend as an adventure-seeker who loves to climb precarious places, spent about two hours early Sunday atop the symbolic and unfinished 1,776-foot tower.

Joe Dunne, security chief of the bistate port agency that has jurisdiction over the lower Manhattan building, said officials "take security and these types of infractions very seriously."

The Weehawken, N.J., teen, who was being held without bail after an arraignment Monday on a criminal trespassing charge, told police he simply walked around the construction site and found a way through the scaffolding around 4 a.m., according to a court complaint. He squeezed through a 1-foot opening in a fence, said Joe Pentangelo, a spokesman for the Port Authority of New York and New Jersey police. Casquejo said he climbed to the sixth floor, rode an elevator to the 88th and took the stairs to the 104th. There, he got past an inattentive guard, a private contractor employee who has since been fired, Pentangelo said. Casquejo told police, "I went to the rooftop and climbed the ladder all the way to the antenna." He was arrested in the building by a Port authority police officer and his camera and cellphone were seized, Pentangelo said.

The security guard's former employer, the Durst Organization, said it had reviewed its security at 1 World Trade Center and was working "to improve coordination" with the Port Authority, which owns the site.

If Casquejo were looking for bragging rights, he could scarcely have picked a more prominent building. Throughout its rebuilding since the Sept. 11, 2001, terrorist attacks, the building once known as the Freedom Tower has been enmeshed in elaborate security plans. Ultimately, plans call for a $40 million system of barriers and checkpoints around the 16-acre trade center, which includes several towers, the Sept. 11 memorial, a transit hub and other features.