Authorities have determined that a suspicious U-Haul parked across the street from a federal courthouse in West Palm Beach, Fla., for about a day wasn't a threat, but they impounded the vehicle.

Hundreds of workers who were evacuated from the courthouse and nearby buildings, which include a U.S. Post Office and the Florida Health Department, were cleared to return to their offices.

Police spokesman Chase Scott said the van with Utah license plates had been parked near the courthouse for at least a day before it caught the attention of a bomb-sniffing dog. Scott said he didn't know why the van -- which was determined to be clear of explosives -- alerted the animal.

But given the date, the location, the fact that it was a rental vehicle, has (anti-law enforcement) graffiti all over it, Scott told The Palm Beach Post. We were taking precautions.

The evacuation comes amid heightened security at federal facilities and major transit hubs after the CIA received a specific, credible but uncorroborated intelligence report that terrorists linked to al-Qaeda may have been plotting a vehicle-based attack to coincide around the anniversary of the 9/11 attacks.

Law enforcement and transportation officials across the country have increased security and cautioned American citizens to be on high alert for any suspicious activity. On Sunday, fighter jets were dispatched to shadow two commercial flights into New York and Detroit after passengers were observed behaving suspiciously. All passengers were interviewed and subsequently released.