Taren Stinebrickner-Kauffman says she's been told an out-of-date manifesto was a key element in the case against him

The partner of late internet freedom activist Aaron Swartz has accused the US Department of Justice of "dragging its heels" over an investigation into his prosecution after it emerged that his political beliefs played a role in its case against him.

Taren Stinebrickner-Kauffman said she was "angry and really upset" when she learned from congressional staffers from the government oversight committee that a document written by Swartz five years ago was a key element in his prosecution.

"I was surprised that the Department of Justice would be so bold, that their motivation was so political," Stinebrickner-Kauffman said. "That it wasn't just one prosecutor run amok, that it was about Aaron's political views."

Speaking to the Guardian from Sydney, Australia, where she is on compassionate leave from her job as executive director of corporate watchdog, SumOfUs, Stinebrickner-Kauffman said she learned of the document's role in the prosecution after a friend posted the story, first reported by the Huffington Post, on Facebook.

The news made her more angry than she had been since Swartz died, she said. "This is not the Department of Justice, it's the Department of Vengeance. If you look at the Department of Justice they are not interested in admitting their mistakes, they are interested in covering their asses."

Since Swartz took his own life, on 11 January, months ahead of a trial in which he faced 13 felony charges and a prison sentence, Stinebrickner-Kauffman and Swartz's family have accused prosecutors of overreach for an alleged crime with no victims and of being complicit in his death.

Swartz was accused of using the MIT computer system to access too many academic articles from a digital library called JSTOR, with the intention of making them freely available. JSTOR chose not to pursue charges against him, after the activist returned all content to them and ensured it would not be used or distributed. But prosecutors indicted him on 13 felony charges. If found guilty, he faced up to 35 years in jail, and although lawyers said plea bargain negotiations were under way before his death, his request for a solution which did not involve a prison term was denied.

Last month, representatives Darrell Issa and Elijah Cummings, respectively the leading Republican and Democrat on the House oversight committee, announced they would investigate prosecutors handling of the Swartz case.

A congressional aide told the Guardian that, in a recent meeting for staffers on the House oversight committee, representatives from the Justice Department said the Swartz document – which Swartz called the Guerilla Open Access Manifesto – was a factor in establishing "malicious intent" to download documents on a large scale. The aide also confirmed that some of the staffers got the impression at the meeting that prosecutors believed they needed to convict Swartz for a felony in order to justify bringing the charges in the first place.

The aide said: "We felt the briefing for the committee was quite thorough". However, it is understood the committee has asked the Justice Department to speak to the US attorney in Boston, Carmen Ortiz. Ortiz has said she believed the prosecution was "appropriate" but her standing has already suffered from a backlash to what is seen by lawyers, internet activists and supporters of Swartz as a disproportionate and overzealous prosecution.

A White House petition demanding the removal of Ortiz garnered well over 25,000 signatures, reaching the level which guarantees a response from the Obama administration.

The Manifesto, which Swartz wrote five years ago, argued that the world's scientific literature was being "locked up" by a handful of corporations motivated by "greed". There was a "moral imperative" to sharing information on the internet, he said, and urged "civil disobedience" in order to achieve it.

"It's time to come into the light and, in the grand tradition of civil disobedience, declare our opposition to this private theft of public culture," he wrote.

"We need to take information, wherever it is stored, make our copies and share them with the world. We need to take stuff that's out of copyright and add it to the archive. We need to buy secret databases and put them on the Web. We need to download scientific journals and upload them to file sharing networks. We need to fight for Guerilla Open Access."

Stinebrickner-Kauffman, who began dating Swartz in 2011, said that the five-year-old Manifesto could no longer be relied upon as an accurate portrayal of his political beliefs years later.

"When you are a voracious active reader in your mid-twenties, your political views change a lot" she said. "My views have. Copyright issues and internet freedom issues were central to his political life many years ago, but when I knew him they weren't. It wasn't that he wasn't interested in them but he was no longer a single issue activist. He was into lots of things, from healthcare, to climate change to money in politics."

She accused the DOJ of ignorance over new technology and of failing to draw a distinction between a "researcher like Aaron and "and someone who is trying to steal credit cards and hack into a database".

"They have decided that the best they can do it have a law that charges anyone with a federal felony, [rather] than have a law that defines crimes in a reasonable way."

Swartz was charged under the Computer Fraud and Abuse Act, a law that has attracted widespread criticism for being too broad. Zoe Lofgren, a Democrat from Silicon Valley, has drafted an amendment to the Act, called "Aaron's law", to prevent similar prosecutions.

"To them, somebody like Aaron was viewed in the same way as they viewed a suicide bomber." said Stinebrickner-Kauffman. "They want to be able to do anything. The Department of Justice has a long history of trying to suppress civil liberties and it has gotten worse since the war on terror started."

In a blog post written the day she discovered the revelation, she said: "the terrifying fact I'm trying to highlight in this particular blog post is this: according to the DOJ's testimony, if you express political views that the government doesn't like, at any point in your life, that political speech act can and will be used to justify making 'an example' out of you once the government thinks it can pin you with a crime. Talk about a chilling effect on freedom of speech."

In a footnote on the post, she wrote that the DOJ had complained her characterisation of the prosecution as "political" was inaccurate, adding: "No argument as to why or how, so colour me unconvinced.

Stinebrickner-Kauffman said that she did not know what Swartz planned to do with the articles and neither did anyone else. The couple did not talk about issues surrounding the charges he faced, partly because he was afraid she would be subpoenaed and partly because "he thought I was a safe haven".

"You want to be able to have part of your life where you don't have to talk about the most terrifying part of your life."

She said that part of the reason it has made her so angry is that she worked to get President Barack Obama elected in 2008. She wrote: "I helped these people get in power. And then they drove the man I loved to suicide because they didn't like something he said once."

Stinebrickner-Kauffman said that she is anxious for the investigation to be completed, so that she will learn the truth and urged the DOJ to disclose all they knew.

"I want them to do it in a way that's designed to find justice, to find the truth. Everything we've seen so far is for them to cover their asses.

"It's definitely changed my attitude towards the Department of Justice, but we don't know enough yet. The Department of Justice has been dragging its heels on the investigation. I don't know who to blame yet and I really, really want to know."