Republican paramilitaries have obtained the information that leaked in a Police Service of Northern Ireland data breach, according to the force’s chief constable. Simon Byrne said on Monday that he believed dissident republicans had the dataset that mistakenly disclosed the personal details of more than 10,000 officers and staff last week. “It’s therefore our planning assumption that they will use this list to generate fear and uncertainty, as well as intimidating or targeting officers and staff,” Byrne told a press conference at the PSNI’s Belfast headquarters. The admission deepened anxiety that paramilitaries would use the data to threaten and attack officers, and deter police recruitment, especially among Catholics. Liam Kelly, the chair of the Police Federation of Northern Ireland, which represents officers, said the statement was worrying but not a surprise. He urged members to “exercise maximum vigilance” on and off duty. “We must do all we can to frustrate and prevent attacks on our colleagues and their families. Our men and women are resilient and resourceful. They must call on all their training and professionalism to counter this ugly consequence of a monumental data breach,” he said. “We have to be strong and determined to do everything we can to minimise risk and that means varying the routes we take to and from work, changing routines and reassessing our personal security.” Byrne said the force was working around the clock to assess and mitigate risks, but the volume of personnel involved made it difficult to “triage and ring people back”. He rejected media reports that some officers and staff had quit the PSNI. The PSNI had been strongly supported by cyber specialists from across the policing system, he said. “We have measures in place to reassure and advise our workforce of what this risks means for them. We will continue to liaise with the policing board and the UK government, as well as other partners, as we develop our response to this matter.” Earlier on Monday, a document purportedly from the data leak appeared on a wall beside a Sinn Féin office on Falls Road. A threatening message and a photograph of Gerry Kelly, the party’s policing spokesperson, appeared alongside the poster. “This is a very obvious attempt by dissident republicans to intimidate me,” said Kelly, a member of the Stormont assembly. “Even more sinister, this is a very public indication that the dissidents do have access to the sensitive information in the data leak document. It therefore represents a very real threat to the officers and the civilian staff involved.” Sinn Féin would not be intimidated by dissident groups that had negligible support and offered nothing but disruption and threats in an attempt to gain relevance, he said. The PSNI apologised for “systemic” failures that led to the names, ranks and departments of staff appearing briefly online on 8 August in a bungled response to a freedom of information request. There was an earlier data breach on 6 July when a police-issue laptop and documents identifying 200 officers and staff were stolen from a private vehicle. The PSNI was notified on 4 August, a time lag that has fuelled criticism of the force’s response to the leaks. Police representatives fear the crisis will especially affect the recruitment of Catholics, who are deemed most vulnerable to paramilitary attack. An officer from a nationalist background said in a letter to the Guardian: “For 18 years, I’ve relied on my family to help me protect my identity. I’ve denied myself a social life and gave up sports I loved. I can’t stay in touch with old friends on social media as I need to avoid the footprint. Now, this is all taken away at the click of a button.” About 3,000 officers have contacted the police federation about a potential legal case for damages. Lawyers have estimated that the eventual bill could be tens of millions of pounds. Nearly 2,000 officers and support staff have expressed personal security concerns because of the leaks. They have given republican paramilitaries such as the New IRA a powerful tool to demoralise and collect intelligence on officers and their families for years to come, according to security experts. Dissident groups have staged sporadic attacks in Northern Ireland over the past decade, including shooting and wounding DCI John Caldwell in February.