Professor Susan Feiner thought her medical marijuana was legal in New Jersey. She was wrong.

The registered medical marijuana patient from Maine was handcuffed, searched and held in a cell at Newark Liberty Airport for several hours under threat of arrest last week after the TSA detected a few grams of cannabis in her checked luggage.

The rare and disturbing incident highlights the growing clash among a patchwork of states' medical cannabis laws and enduring federal prohibition.

TSA says it is not looking for marijuana in your luggage. Yet, as agents slip their fingers between the clothes and toiletries of millions of travellers, they invariably encounter some personal amounts of America's most popular flower.

The consequences for consumers and registered patients depends on the airport.

"TSA does not detain passengers, only law enforcement has that authority," said TSA regional spokeswoman Lisa Farbstein.

She also referred to the TSA website that specifies medical marijuana is banned from checked and carry-on bags. Yes, even in states where it's legal.

TSA agents don't alert Homeland Security over a joint or a dime-bag. Instead, the weed and the unlucky passenger are handed off to local police.

On June 11, Feiner was waiting to board a United Airlines flight back home to Portland, Maine.  She had been in New Jersey visiting for her mother's 89th birthday.

Handout
Susan Feiner, a professor at the University of Southern Maine. (Handout)

Feiner, 63, is a professor of women's and gender studies, as well as economics, at the University of Southern Maine. As the passengers lined up at Gate A20, a TSA agent suddenly called Feiner's name over the intercom. It was the beginning of an all-night ordeal.

Her checked suitcase had been selected for a random search after it was tagged and out of her possession. That's when TSA found less than ¼ ounce of medical cannabis.

Feiner was afraid and confused when the agent handed her over to three Port Authority of New York & New Jersey (PAPD) police officers.

She was told to produce identification as her fellow passengers filed past onto the airplane.

"I was laying my credit cards and business cards out on a chair. I was shaking," said Feiner.

"That's when one of them said 'I sure hope you have your New Jersey medical marijuana card.'"

Feiner immediately stated that she does have a card from Maine. At the time, PAPD officers seemed to think this might help her avoid arrest.

But New Jersey's 2010 compassionate use law does not honor cards from other state programs.

Feiner wasn't aware of that because her state does have a form of reciprocity for visiting patients. Moreover, voters in Maine fully legalized marijuana for adults in a 2016 ballot initiative.

One of the PAPD officers then started asking questions in a firm tone: What do you do for a living? What are you doing in New Jersey?

"That's when I started crying," said Feiner. "The absurdity of this was more than my brain could handle. I started being a little hysterical, but realized if I got weird, the situation could get worse."

A female PAPD officer calmed her down, and Feiner overheard another officer talking to a superior via phone. He was asking how to proceed.

The PAPD officer approached and said, "I'm very sorry about this but we have to arrest you."

He took out the handcuffs and Feiner was aghast. She asked not to be cuffed. They said it was protocol.

"They took me into the boarding tunnel and had me face the wall with my hands up. Then they did that thing where they knocked my feet further apart," said Feiner.  "Being handcuffed was quite uncomfortable, I have a medical marijuana [recommendation] for pain."

An old injury in Feiner's shoulder began throbbing and a few minutes later, her back began to spasm. Obviously she wouldn't be allowed to take a few puffs of medical cannabis, and now all of the other prescription medications in her purse were off-limits too.

Feiner's cellphone was taken and she was walked down the boarding tunnel out onto the tarmac. Three PAPD cruisers, lights blazing, were waiting.

Everyone in the terminal had a bird's eye view of the professor being treated like a hardcore criminal or security threat.