One expert said people with pre-existing mental health conditions were particularly at risk of distress.

Photograph: golibo/Getty Images/iStockphoto Livestreaming an execution could cause post-traumatic stress disorder in the viewing public, mental health experts have said after the US confirmed plans to broadcast the death by firing squad of a man who shot dead 13 unarmed soldiers.

The US defence secretary, Pete Hegseth, said on Thursday that Nidal Hasan, a former army psychiatrist who killed 13 people at Fort Hood in 2009, would be executed by firing squad on 3 December. Officials later confirmed it would be livestreamed. It is believed it will be the first public execution in the US in almost a century.

Hegseth said: “It was a radical Islamist attack in uniform inside the ranks, and so he’s going to get a firing squad of soldiers, as it should be, and we’ll make sure that people are able to watch it, that it’s public, because people need to understand there’s serious consequences for these types of things.”Experts said watching the event could result in lasting harms for viewers.

Dr Simon Duff, a forensic psychologist at the University of Nottingham in the UK, said that while there was not a lot of research into the impact of watching an execution, studies examining journalists’ experiences of witnessing violence suggested evidence of trauma response.

“Certainly if people are not resilient they probably shouldn’t [watch], but if they have some underlying psychological driver towards viewing an execution they will likely do so regardless of the potential for harm,” Duff said.

“Recent events suggest that executions do not always go to plan,” he said, referring to the failed execution by lethal injection of Christa Pike in the US. “That possibility needs to be acknowledged, and the likelihood that this would increase the trauma for viewers.”Neil Greenberg, a professor of defence mental health at the Institute of Psychiatry, Psychology and Neuroscience at King’s College London, said trigger warnings did not routinely lead to people avoiding media they may find distressing.

“Instead they are likely to suffer anticipatory anxiety before the ‘trauma’ is shown and some people will then become distressed,” he said.

“It is likely that people who have pre-existing mental health problems will be especially at risk of becoming distressed or experiencing a deterioration in their mental health.

“My advice would be not to watch the event at all and I would hope that mainstream media would not transmit highly traumatic images [or] sounds.”Public executions were once common around the world, including in the UK, but only two out of 17 countries that carried out executions last year did so publicly.

Prof Rosalind Crone, an historian at the Open University in the UK, said: “Public execution in England and Wales was abolished in 1868 because of persistent and increasing concern about the effects of watching hangings on the crowd.

“It was clear that public execution did not have a deterrent effect, in fact, sometimes the opposite.”Crone said crowds were increasingly made up of people missing work to attend, and people who were drunk, rowdy and celebratory.

“Charles Dickens sums up the increasing objections well in his letter to the Daily News in 1846: ‘No sorrow, no salutatory terror, no abhorrence, no seriousness, nothing but ribaldry, debauchery, levity, drunkenness and flouting vice in fifty other shapes’,” she said.

“Public execution – and execution full stop – does not work as a deterrent,” Crone added. “If anything, the use of a medium like video broadcast will likely generate new, potentially more dangerous problems in managing crowd responses and reactions.”