She is, in the eyes of the law, America's most dangerous eco-terrorist: a self-confessed serial arsonist who resorted to fire and destruction to register her opposition to the fur industry and genetically modified crops.

But to those who know her and to some legal experts, the 22-year jail term handed to Marie Mason, 47, is a consequence of America's preoccupation with terrorism in the post-9/11 world.

She is serving the longest sentence of any convicted animal rights or environmental militant, including several activists responsible for greater destruction.

"It is obvious the government is trying to send a message – to have a chilling effect, not only on my action, which of course transgressed the laws, but also on 30 years of above-ground actions in the environmental rights spheres," Mason told the Guardian in her first interview since she was sentenced last month.

Mason was convicted on the evidence of her fellow arsonist and ex-husband, Frank Ambrose. He was jailed for nine years.

"It's very, very sad. These are karmic things that Frank will have to deal with on his own," she said.

The explosive fire Mason and Ambrose set at Michigan State University on 31 December 1999 caused nearly $1m (£680,000) of damage to buildings and equipment, but no death or injuries. The target was the office of the director of a genetically modified crop research programme into moth-resistant food crops for Africa, funded by the US Agency for International Development and the biotechnology company Monsanto.

Professor Daniel Clay, who worked at the institute in 1999 and is now the director, said the attack had a severe impact on the staff. "It really was a shock," he said. "It was a very difficult period for all of us. People were frightened and we asked ourselves how close did this come to physically harming someone."

However, Mason's lawyer, John Minock, who filed an appeal against the sentence last week, argues that 22 years is excessively harsh. Mason got a much longer sentence than several militants recently convicted of setting fire to logging camps and vehicles in Oregon and Washington states – including Stanislas Meyerhoff who received 13 years for setting 11 fires and causing $30m in damage.

"Giving her a 22-year sentence is like using a cannon to shoot a mouse," Minock said. "She is a 47-year-old, mild-mannered woman with no previous criminal record other than trespassing."

The FBI had singled out militant environmentalists and animal rights activists as domestic security threats even before the 9/11 attacks. Since then, the courts have used domestic terrorism laws to stiffen the punishment for politically inspired violence.

Mason is a prime example. "We are definitely seeing more severe sentences post-9/11, no doubt about it," said Heidi Boghosian, the director of the National Lawyers Guild. "We have seen a trend of using the terrorist label and federalising a lot of criminal activities that would have gotten a far less stringent sentence before."

Lauren Regan, an Oregon lawyer who defends environmental militants, calls it the "green scare".

To those in Mason's home city of Detroit who know her, her elevation to the ranks of America's most dangerous criminals came as a shock. A fixture in activist circles, she was bright and charming, but unfocused – a woman who had an advanced degree in chemistry but lived near the poverty line.

They saw her as a doting mother to her adult son and teenage daughter, a soft touch who took in stray dogs and named them after revolutionary heroines, an amateur folk singer and a passionate supporter of various causes. But not, they say, the organiser of a series of attacks.

"She is one of those people that, whenever there would be a demonstration, she would be there," said Peter Werbe, a Detroit broadcaster who has known Mason for 20 years. "I don't think she ever rose to prominence as a figure in the city."

A long-time acquaintance said: "If you look at the court documents, Frank is always the one lighting the fire, she is always the one spray-painting the wall. That, in a nutshell, is who she is. Marie is always the support staff."

Mason met the man who was to become her third husband in 1998. Ambrose, now 34, was well-known among forestry activists in the mid-west and was leading a workshop for activists.

Mason says there was an instant attraction. The two were soon living together and married, although the relationship was troubled.

At about 9pm on New Year's Eve 1999, the couple entered an office in the Institute of International Agriculture at Michigan State University and doused it with petrol.

The arson, by Mason's own account, was botched. A fireball set her hair on fire, forcing the couple to run before she even managed to write her slogan, "No GMO".

The next day, the pair set fire to a logging camp. She has also admitted to burning boats belonging to the owner of a mink farm.

When asked whether the fires might have terrified staff and students at the university, she said: "It was intended as an enlightenment moment that people would see what is going on beneath the surface."

Clay argues his institute's research was aimed at creating a more sustainable agriculture, an ambition he believes should be shared by environmentalists. "It was most important to us that the perpetrators were caught and that justice was served," he said.

The couple nearly got away with it but were caught in March 2007. By then, the two had split up.

Ambrose, who was cleaning out his possessions, left gas masks, fuses, maps and explosives in a rubbish dump. The material was discovered by a man who called the authorities. Eventually, the gas masks led the FBI to Ambrose, who agreed to turn informer. He wore a wire and gave Mason a mobile phone, to help the FBI monitor her conversations.

Mason, in her jail cell, has often thought about those talks. "I did think that some of our conversations were very strained and strange but I attributed that to the fact that Frank was a very nervous person and he was under a lot of pressure," she said. "I was the last to know that Frank was unreliable."

Additional reporting by Damian Carrington.

• This article was amended on Wednesday March 25 2009 to replace the headline used with that which appeared in the paper version of the article.