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.

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.”