Four people have been arrested as part of an investigation into a New IRA plot to send letter bombs to army recruitment centres as well as other targets across England.

The Police Service of Northern Ireland confirmed on Wednesday it had detained the four in Derry in connection with the discovery of parcel and letter bombs at sorting offices in February. The four arrested are two men aged 35 and 46, and two women aged 21 and 44.

They are also being questioned about a letter bomb plot in Northern Ireland where the targets included the office of the secretary of state, Theresa Villiers. A parcel bomb was also sent to the public prosecutions office in Derry.

A PSNI spokesman said all four were being questioned at the police's serious crimes suite in Antrim, on Wednesday.

The republican terror alliance known as the New IRA admitted responsibility for a series of parcel bombs sent to army recruitment offices across England.

Scotland Yard has confirmed the New IRA used a recognised code word in claiming responsibility for sending the suspect packages to military careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough.

Earlier the terror group had sent similar packets to Aldershot, Reading and the RAF careers office in Chatham, Kent.

A number of the packages had Dublin postmarks on them and were thought to contain low-grade explosives. The shopping centre was temporarily evacuated, while cordons were placed close to all the offices where packages were found. Ministry of Defence bomb disposal units were also called in to deal with the devices.

The parcel bombs marked a fresh phase in the New IRA's armed campaign by targeting mainland Britain. Before February's incidents, dissident republican organisations had not struck at targets in England for almost a decade.

In Ireland, a series of arrests have been made over the past two years among all three anti-process republican groups – the New IRA, the Continuity IRA and Óglaigh na hÉireann – with security forces on both sides of the Irish border thwarting at least half a dozen attempts to transport bombs and mortar devices into Belfast and Derry.

A week ago the New IRA put on a show of strength at the funeral of Belfast republican veteran Tony Catney, with masked men in paramilitary uniforms flanking his coffin openly on the streets of the city.

The PSNI has since vowed to use new forms of facial recognition imagery to arrest the masked pallbearers at Catney's funeral. This technology can in theory "unmask" those hiding their faces at such occasions.

On the eve of the funeral, the New IRA also filmed and distributed a YouTube video of three men armed with AK-47-type rifles, firing shots in the air over a portrait of the republican leader in his honour.