(CBS) — It was stunning for those who watched Thursday night as federal agents investigated a possible nuclear threat at Chicago’s Ogilvie Transportation Center.

CBS 2’s photojournalist Lana Hinshaw-Klann happened to be at the scene and used a cell-phone camera to record agents in action. Reporter Dave Savini looks into what agents were looking for and what they found.

Sources say the agents were members of the elite TSA VIPR team on the 5:04pm Union Pacific West line. They were carrying hand-held nuclear-detection devices that picked up a reading.

VIPR teams were created after the 2004 bombing of a train in Madrid, Spain, to protect U.S. transportation.

At the Ogilvie station, officers held the train and searched for a person or bag that posed a potential nuclear threat.

Jerry Jones, a Chicago lawyer, was heading home on that train. He says the federal officers narrowed the trouble to the area where he was sitting.

“I had no idea I was the center of the activity,” he says.

The special security team must have picked up on him as he entered the station and walked up the stairs, Jones says. Little did he know a nuclear stress test he had at a hospital earlier in the day had set off silent alarms and sent security scurrying.

The TSA team passed by him several times before ending up on his train car. Finally, he got a clue when an agent questioned the man right next to him and asked, ‘Sir, do you have an explanation as to why I am getting a high isotope reading on your bag?’”

“The fellow’s jaw dropped,” Jones said.

Once the agent said the word “isotope,” Jones says he realized he was the one they were looking for. He raised his hand to say he had a nuclear stress test.

The tests can leave patients emitting radiation for some time. After showing identification and proof of the nuclear test, Jones and the other passengers were allowed to go on their way.

He says he’s satisfied with the way authorities acted, “knowing there are people on the lookout for this type of thing,” and was pleased with the way officers and passengers behaved.

Patients undergoing nuclear testing can request a card they can give to security if they travel afterward. Doctors have done this for air travelers.