Pedestrians walk in front of the 1 World Trade Center building, left, in New York.

NEW YORK - 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 as concerns swirled about the audacious breach at what is supposed to be one of the world's most secure sites.

Justin Casquejo, a New Jersey 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 as-yet-unfinished 1,776-foot tower, authorities said. He apparently just wanted bragging rights and perhaps some photos, but the alleged escapade stirred what-ifs about the notion of someone with a more sinister agenda infiltrating the nation's tallest skyscraper.

"Obviously, it was shocking and troubling," said Mayor Bill de Blasio, "and I don't know how possibly it could have happened."

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

Casquejo was being held without bail after an arraignment Monday on criminal trespassing. His lawyer, Pamela Griffith, declined to comment. Nobody answered the door Thursday at his Weehawken, N.J., home; an effort to reach him through someone who answered a possible phone number for him wasn't immediately successful.

Casquejo 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 one-foot opening in a fence, said Joe Pentangelo, a spokesman for the Port Authority of New York and New Jersey police. The authority owns the site.

The court complaint quotes Casquejo saying that 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 security 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," according to the complaint.

WABC-TV reported that he took pictures from the top of the building; authorities said they were still trying to determine the teen's motive. He was arrested on the site, and his camera and cellphone were seized after authorities obtained a search warrant, Pentangelo said.

Patrick Flores, an 18-year-old neighbor who grew up with Casquejo, described the teen as "a really good kid" who has always been highly interested in adventure.

"He was always the one climbing the cliffs, doing something stupid," Flores said, referring to the cliffs on which Weehawken sits, across the Hudson River from Manhattan, with clear views of the World Trade Center and the rest of the Manhattan skyline. "But that was him - that was his life."

"I've seen him fall and hit his head and get up and walk away like it was nothing," he added.

Flores said Casquejo had recently become interested in parkour, the extreme sport that combines elements from martial arts, gymnastics and rock climbing and has become popular thanks to YouTube videos of acrobatic athletes vaulting over obstacles such as park benches, trees, guardrails and buildings.

If he was looking for bragging rights, Casquejo couldn't have picked a better building. Throughout its rebuilding since the Sept. 11 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.

Most of the planned security has yet to be built, but some nearby residents have challenged the plans as overbearing. In a lawsuit last fall, residents said the security measures would turn their neighborhood into a fortress-like environment "as impervious to traffic as the Berlin Wall."

City lawyers defended the security plans as necessary, and the city said the measures were as unobtrusive as possible. A judge dismissed the case last month; the residents are not pursuing an appeal but are pressing their concerns to local officials, their lawyer, Daniel Alterman, said Thursday.

To visitor Steve Murphy, the plans seemed adequate, despite Casquejo's alleged breach.

"I think it's just a one-off thing. ... It's nothing that poses a threat to American security," said Murphy, of York, England, who added that he'd seen danger firsthand during 22 years in the British army. "It's not a concern, at least in my mind."