The World Trade Center was destroyed just days after a heightened security alert was lifted at the landmark 110-story towers, security personnel said yesterday.

Daria Coard, 37, a guard at Tower One, said the security detail had been working 12-hour shifts for the past two weeks because of numerous phone threats. But on Thursday, bomb-sniffing dogs were abruptly removed.

"Today was the first day there was not the extra security," Coard said. "We were protecting below. We had the ground covered. We didn't figure they would do it with planes. There is no way anyone could have stopped that."

Security guard Hermina Jones said officials had recently taken steps to secure the towers against aerial attacks by installing bulletproof windows and fireproof doors in the 22nd-floor computer command center.

"When the fire started, the room was sealed," said Jones, who was in the command center when explosions rocked the building. "Flames were shooting off the walls....We started putting wet towels under the doors. The Fire Department unsealed the door and grabbed us by the hand and said, 'Run!' "

Security worker Diane Easton said she was out front writing tickets when notice of "a plane flying too low" placed security personal on alert.

"The plane went right through the building...and debris started falling everywhere, then 15 minutes later the second plane," Easton said.

Nancy Joyner said several police officers were knocked down and then were stampeded in the ensuing frenzy.

"People were flying out of the windows; there was nothing we could do," Joyner said. "People were pulled from the top floors by the wind tunnel created."