(CBS 3) PHILADELPHIA   With mass transit security on the minds of many commuters these days, CBS 3 has learned that SEPTA and Philadelphia Police have a major investigation going on. The investigation involves suspicious activities at one of the city's busiest transit hubs at the Frankford Transportation Center. Several riders at one of SEPTA's busiest depots expressed their concern after learning two men, impersonating SEPTA workers, were spotted early Sunday morning, inside the nearby depot, trying to get a closer look at the buses mechanical components. "With the terrorist stuff going on, it is real scary," said a SEPTA rider.

According to an official SEPTA report, obtained by CBS 3, "the males were wearing dark blue uniforms, one of them had a SEPTA patch on his sleeve, they said they were from the I.G.'s (inspector generals) officer and wanted to see the undercarriage of a bus." SEPTA credits an alert supervisor with spotting the two fake employees, who also flashed a fake SEPTA I.D. card. The pair disappeared before police arrived, but minutes later, the depot manager received a mysterious phone call by a man claiming to be a SEPTA official, demanding to know why the pair had been denied access.

The call, like the workers, was bogus. "With all the 9/11 terrorist stuff going on, it is a little scary," said rider, Shariff Paxton.

SEPTA officials, confirming a joint investigation with Philadelphia police into the suspicious activities have now issued an alert to workers city-wide which reads in part: "If the suspicious individuals are asking to inspect SEPTA buses, trains, or facilities, summons the local police by calling 911.