Rain clouds over the launch pad forced NASA to call off Tuesday's liftoff of space shuttle Endeavour on a flight to deliver a new crew to the international space station.

It was the latest delay for Endeavour and its seven astronauts, held up last week by space station trouble. Launch managers said they would try again Wednesday.

Security was at an all-time high, especially during the final hours of the countdown. The clocks were halted at the five-minute mark once it became clear that weather would not cooperate.

Endeavour was supposed to blast off last Thursday, but had to wait for the space station's residents to clear a jammed docking mechanism. Monday's spacewalking repairs allowed an unmanned Russian supply ship to finally latch itself firmly to the orbiting outpost.

Besides the dynamic weather, the launch team also had to contend with a set of hand rails inadvertently left at the launch pad.

With time running out, NASA rushed a three-person red team to the pad to remove the temporary rails, which should have been taken down Monday. A quick visual sweep turned up no other stray equipment, and the countdown proceeded — until the clouds and rain moved in.

Engineers feared the hand rails, located near Endeavour's engines, would blow away at liftoff and possibly damage the shuttle.

Launch director Mike Leinbach urged his team to "stay focused" and not be distracted by the last-minute flurry of activity.

To guard against terrorist attacks, fighter jets and helicopters kept watch over the launch pad, catching at least one violator, a helicopter pilot who strayed into the no-fly zone surrounding Kennedy Space Center.

The patrols increased — and included at least one Humvee with a .50-caliber machine gun — after Endeavour's tank was filled with more than 500,000 gallons of fuel.

Signs of patriotism were everywhere for NASA's first wartime shuttle launch. Space center workers waved small flags as the astronauts headed to the launch pad, and some wore neckties with flag designs.

For the first time in 20 years of space shuttle flight, journalists were kept away when the crew left for the launch pad. Even space center employees had to stand a few hundred feet away behind a barricade.

Space center roads, normally filled with spectators during launches, were closed. The no-fly zone extended 35 miles around the launch pad, much farther than usual, and boats had to stay at least three miles offshore.

As a high-flying salute to victims of the Sept. 11 terrorist attacks, Endeavour was carrying 6,000 small U.S. flags for the families of all those killed. Also on board: a Marine Corps flag from the assaulted Pentagon, New York police badges and patches, and a New York fire department banner.

Aboard the space station, commander Frank Culbertson and his two Russian crewmates had already pulled out their cameras to record the liftoff of Endeavour, their ride home. Culbertson and cosmonauts Vladimir Dezhurov and Mikhail Tyurin have been living aboard the space station since August.

Their replacements, two Americans and one Russian, will stay until May.

During Endeavour's 11-day flight, the shuttle crew will drop off thousands of pounds of supplies, perform a spacewalk and help the new station crew move in and the old one move out.

American astronauts Carl Walz and Daniel Bursch and Russian cosmonaut Yuri Onufrienko will be the fourth crew to live aboard the space station.