Environmental campaigner Emma Sheppard set up traps outside police station near Bristol, damaging three vehicles and ‘risking lives’, judge said

An environmental activist faces jail for putting the lives of police officers in danger by successfully setting up a home-made trap designed to take patrol cars out of action.

Emma Sheppard brought three cars to a juddering stop by puncturing their tyres with the crude “stinger” device made of plywood and nails that she had positioned outside a police station near Bristol on New Year’s Eve.

Emma Sheppard, who has been convicted in Bristol of damaging police cars with a stinger device. Photograph: Public Domain

Sheppard’s conviction is the first following an arrest by detectives from Avon and Somerset police’s Operation Rhone, which is probing more than 100 attacks on establishment targets including police stations, banks and politician’s cars by suspected anarchists in and around Bristol.

Sheppard is well known within green activist circles and is one of the campaigners who was found guilty of trying to shut down the Ratcliffe power station in Nottinghamshire in 2009 but whose conviction was quashed following the revelations that the group had been infiltrated by the undercover police officer Mark Kennedy.

At a brief hearing at Bristol crown court on Tuesday, Sheppard, 33, appeared via video-link from Eastwood Park prison in Gloucestershire.

Wearing all black, she spoke only to confirm her name and to plead guilty to damaging property and being reckless as to whether her actions endangered lives.

Judge Martin Picton told Sheppard, who is from the Easton area of the city – a neighbourhood associated with Bristol’s radical scene – that he would have to consider public protection issues when sentencing her next month.

Ordering a pre-sentence report, he told Sheppard: “The court will have to know a lot more about you to determine what is the right sentence. It will inevitably be a custodial sentence.”

The facts of the case were not given in court, but the Guardian understands that on New Year’s Eve Sheppard placed a home-made stinger made of nails and plywood across a road close to Concorde House in Emersons Green, a police base to the east of the city centre. Police and armed forces typically use stingers to stop suspects’ cars and to defend road blocks.

Three police response vehicles had their tyres punctured as they left the police station together to deal with an incident. No officers were hurt.

Avon and Somerset police regard the guilty plea as significant because it is the first conviction credited to Operation Rhone. Detectives from Rhone, which has a permanent team of 10, were called in to investigate Sheppard’s attack because it was considered an assault on the establishment.

In December, for the first time police linked more than 100 arson and vandalism attacks that have been carried out in and around Bristol and Bath over the past four years. The most spectacular arson attack caused £16m of damage to Avon and Somerset’s new firearms centre in August 2013. But other attacks have been carried out on phone masts, railway lines, car dealerships, courts and churches.

Often responsibility for the attacks is claimed on the anarchist website http://325.nostate.net. Police believe a very small group is behind the campaign. Members of Bristol’s long-established and thriving anarchist scene claim the force has unfairly harassed activists because it hates their anti-establishment stance.

A £10,000 reward has been offered over one well-known activist, Huw “Badger” Norfolk. Police have said they want to talk to Norfolk about a vandalism attack on the offices of the Bristol Post in August 2011 – at the time of protests around Britain following the shooting of Mark Duggan in north London – and an arson attack on a phone mast in January 2013 that cut off television, radio and mobile phone signals to thousands of homes and businesses. Norfolk’s location has been unknown to the police since 2011.

In 2010, Sheppard, then living in Manchester, was given a conditional discharge over the Ratcliffe protest. Judge Jonathan Teare told her and her co-defendants: “You are all decent men and women with a genuine concern for others, and in particular for the survival of planet Earth in something resembling its present form. I have no doubt that each of you acted with the highest possible motives. And that is an extremely important consideration.”

The convictions were quashed at the court of appeal the following year after three court of appeal judges ruled that crucial evidence recorded by police spy Mark Kennedy had been withheld. The lord chief justice, Lord Judge, said that the convictions were “unsafe because of significant non-disclosure” of secret surveillance tapes recorded by Kennedy.

No link has been established between Sheppard and any of the other attacks on establishment targets in Bristol, but it is believed that she knew Badger Norfolk.

DCI Andy Bevan, who heads Operation Rhone, said: “These crude homemade stinger devices caused damage to three police vehicles, which were responding to emergency calls on New Year’s Eve.

“Each of these devices had around five large nails sticking through a piece of wood and rendered the police vehicles unusable on what is traditionally one of the busiest nights of the year.

“Emma Sheppard placed these purpose-built devices in the road, knowing full well what the consequences could be.

”They posed a serious risk to our police officers as well as other road users and formed part of a reckless and dangerous plan.”