The widow of an Iranian-Canadian environmentalist who died in a Tehran prison under disputed circumstances has been barred from leaving the country, according to one of her sons.

The family – all of whom are dual citizens of Iran and Canada – were boarding a Lufthansa flight for Canada on Wednesday when Maryam Mombeini, 55, was stopped by security forces and told she was forbidden from leaving the country.

Soon after, her son posted a photo online showing himself and his brother seated in the plane without their mother. “Enough is enough,” Ramin Seyed-Emami wrote on Instagram, noting that both he and his brother would not “stay silent for one second until we are reunited with our mom”.

Mombeini is the widow of Kavous Seyed-Emami, the founder of the Persian Heritage Wildlife Foundation. The group seeks to protect Iran’s rare animals, including the Asiatic cheetah, which ranks as one of the world’s most endangered species, with only 50 remaining.

Iranian-Canadian environmentalist Kavous Seyed Emami Photograph: handout/AFP/Getty Images

Seyed-Emami and several others from the group were arrested in late January. Two weeks after being taken to Iran’s notorious Evin prison, officials said Seyed-Emami, 63, was dead.

Iran’s judiciary said he had killed himself and described him as an agent of the CIA and Mossad who had used the wildlife foundation as a cover to collect information about the country’s missile bases.

Their claims were met with widespread scepticism. His family has been calling for an independent investigation into his death.

On Thursday the Canadian government – which has also pressed Iran for information about the detention and death of Seyed-Emami – demanded that Mombeini be allowed to leave Iran.

“I am outraged to learn that Maryam Mombeini, widow of Kavous Seyed-Emami, was barred from leaving Iran,” Chrystia Freeland, Canada’s foreign affairs minister, wrote on Twitter. “We demand that, as a Canadian, she be given the freedom to return home.”

Canada cut all diplomatic ties with Iran in 2012, expelling Iranian diplomats from Canada and closing its embassy in Tehran. Despite a 2015 campaign promise by Justin Trudeau, Canada’s prime minister, to restore diplomatic relations with the country, the Italian government continues to handle Canada’s interests in Iran.

Ramin Seyed-Emami – a well-known singer in Iran – said his family had decided to leave Iran after persistent harassment and threats had left them living in a “state of constant terror”.

The family had been under pressure to stay silent about the death of Seyed-Emami, he added. “My brother and I are followed and under surveillance everywhere we go,” he said in a statement sent to journalists. “The authorities told our lawyers to tell the brothers ‘to shut up or we’ll shut them up’.”