Travelers and commuters across Atlanta, whether at the airport or on MARTA, are already seeing heightened security as the 10th anniversary of Sept. 11 approaches this weekend.

U.S. counter-terrorism experts revealed a credible but unconfirmed al-Qaida threat to set off a car bomb on bridges or tunnels in New York City or Washington, sites of terrorist attacks 10 years ago.

It was the first word of an "active plot" timed to coincide with the commemoration of 9/11, which killed nearly 3,000 people.

While metro Atlanta was not mentioned in the threat announcement, federal and local transportation agencies and authorities are on heightened alert.

Officials at Hartsfield-Jackson International Airport, the busiest airport in the country, are treating the threat seriously, according to Jon Allen of the Transportation Security Administration.

"As with any significant day or peak travel period, passengers may notice an increased security presence at airports and mass transit systems," Allen said. "We ask that passengers be partners in security and report suspicious behavior to authorities."

The TSA has spent more than $100 million in security upgrades at the airport since 9/11.

"No single layer of security might be impenetrable. That's why you put a whole lot in place,” Allen said.

MARTA customers should expect to see more law enforcement officials and K-9 teams inside stations. Security announcements will be made every 15 minutes on trains.

Officers will be giving out fliers to remind riders to be alert and to tell authorities about any suspicious activity. They also will be wearing traffic vests so they are easier to spot.

Last year, MARTA police officers trained with federal, state and local agencies to prepare for terror threats. The transit agency said it will work with the Department of Homeland Security, TSA, FBI and other agencies to coordinate efforts.

A team of Homeland Security agents with a K-9 unit was also at the Amtrak station in Midtown on Friday.

Burnard Leander Hunter was at the station waiting for the Crescent train to New Orleans. When he arrived he said seven Homeland Security officers were present.

“As long as I see Homeland Security here I have no problems. I have no worries whatsoever,” said Hunter, co-owner of Coyote Coach Express Inc.

Danielle Johnson was at the station also traveling to New Orleans to meet her father and brother.

“I think it’s good they are worried about our safety, but I do feel that if anything does happen it would not happen here at Amtrak,” Johnson said.

First responders will also benefit from an upgraded local communication system. Georgia Emergency Management said the system has been upgraded since 9/11, allowing responders from different jurisdictions to communicate with one another during a crisis.

Atlanta Police spokesman Carlos Campos said the metro area’s largest police department is also ready to react.

“There is presently no credible threat to Atlanta. However, we are mindful of the anniversary and are prepared for any crisis,” Campos said.

--  Staff writers Angel K Brooks and photographer John Spink, Channel 2 Action News and the Associated Press contributed to this report.