A simple piece of paper should serve as a warning alarm for the United States' Homeland Security personnel.

The paper contained what is described as a crude sketch of New York's Grand Central Terminal.

That isn't a concern. Each year, we suspect, thousands and thousands of tourists to the Big Apple snap photographs of the famous terminal.

That's not troubling, either.

The troubling issue is the location where it was found: The home of a suspect in the March 2004 Madrid train bombings.

In addition to the sketch, investigators also found photographs and a drawing of a private building in New York.

Yet another worrisome clue.

New York City Police were quick to point out that by itself the sketches and the photographs mean nothing. They do not contain enough detail to indicate an immediate threat, the security officials said.

And, while that may be true, the fact remains that the evidence found provides proof that terrorists are still plotting against the U.S. homeland.

The discovery should warn Americans that terrorists have considered attacking trains in the United States. Obviously, the terrorists can sense a weakness in our nation's security.

After the Sept. 11, 2001, attacks on the U.S. using airplanes, the federal government threw massive resources at the nation's air security. And, while it was needed, terrorists chose to attack with airplanes in 2001 because they sensed a weakness.

The terrorists realized that our guards were down. "Hijackers?" Americans might have asked before 9-11. "We know how to handle hijackers."

But little could most of us imagine what the terrorists had in mind. They didn't plan simple hijackings. And, we suspect whatever they planned for Grand Central Terminal was not your ordinary bombing, either.

A further troubling issue is that despite the fact that the sketch was found two weeks after the bombings, Spanish officials did not turn over the information found there until December.

Neither the evidence nor the apparent slow delivery of the evidence by Spain should go unnoticed. The terror threat remains all around us. And a simple piece of paper could be the proof.