Police hunting the IRA's most feared killers were last night questioning seven men, five of whom were detained when heavily armed officers and troops swooped on a farmyard near Crossmaglen, in south Armagh, and two arrested later.

During a major security operation, two cars, one fitted with bullet-proof metal and containing masks and radio equipment, were also seized.

It is understood some of those held at Gough Barracks, Armagh, are being questioned about nine single-shot assassinations along the border since 1992, which have left three RUC officers and six soldiers dead.

In follow-up operations across several square miles of countryside police searched for the IRA's so-called Supergun, an American-made Barrett Light 5O, which has a killing range of more than a mile. Both it and a high-powered AK 47 assault rifle have been used in all nine murders, thought to have been carried out by one, or maybe two, gunmen.

The last single-shot death was of Lance Bombadier Stephen Restorick, 23, at a checkpoint in Bessbrook, in February.

Meanwhile, the family of the policewoman shot in the back by the IRA on Thursday, were last night keeping a vigil at her bedside in intensive care.

Mrs Alice Collins, 46, a mother of three, was still seriously ill in Londonderry's Altnagelvin Hospital. She underwent more than six hours of emergency surgery.

Fears were growing of a loyalist reprisal for the shooting.

Mr David Nicholl, of the Ulster Democratic Party, the political wing of the UDA paramilitaries, warned: ''This could be the straw that breaks the camel's back.''

Condemnation came from all sides yesterday, with the Catholic Bishop of Derry, Dr Seamus Hegarty, describing it as ''an act of lunacy''.