WASHINGTON — The federal government is telling emergency managers to be on the lookout for fake emergency and commercial vehicles, as security tightens in the two cities hosting this year’s presidential conventions.

Terrorists could used these “cloned vehicles” to conduct surveillance or to carry out an attack, according to an Aug. 21 bulletin from the Federal Emergency Management Agency.

Cloning a vehicle is easy and relatively cheap, according to the National Insurance Crime Bureau.

For about $2,000 someone can use a computer, color printer, typewriter, barcode label printer, an electric tool for cutting, and an engraving pen to fake vehicle identification numbers, stickers, and titles.

The Secret Service does not have any specific information about these cloned vehicles being used for surveillance or terrorist purposes at the Democratic convention in Denver and the GOP convention in St. Paul, Minn., Secret Service spokesman Eric Zahren said. But the agency is aware of this type of potential threat, Mr. Zahren said.

“We work closely with our law enforcement and public safety partner agencies to identify emergency vehicles in advance for access to secure areas,” he said.

Earlier this month, the Homeland Security Department told its employees that the country is considered to be in a period of heightened alert lasting until next summer. The Aug. 8-24 Beijing Olympic Games, presidential nominating conventions, November elections, and transition to a new administration pose opportunities for terrorists to attack.

The FEMA bulletin cites examples over the past few years in which 18-wheelers were disguised as Wal-Mart trucks and were eventually impounded. For instance, in 2006, Texas authorities stopped a fake Wal-Mart truck that was carrying 3,000 pounds of marijuana and about 450 pounds of cocaine, according to a report on cloned vehicles issued earlier this year by the Florida state intelli gence fusion center.

The bulletin, called an “infogram,” is distributed to emergency management officials across the country. Officials are advised to know how to verify markings on government and military vehicles.

Imaging systems that can see inside trucks as well as radiation detection equipment will be used in both convention cities to prevent anything dangerous from getting near or inside the venues.

Thousands of federal, state, and local law enforcement officials will be working to secure the conventions, as will airport screeners, nuclear weapons experts and intelligence analysts.