LONDON (AP) — Police in Northern Ireland said Monday they were looking into an unverified statement by an Irish Republican Army splinter group claiming responsibility for the shooting of a senior police officer.

A statement purportedly from the dissident group known as the New IRA appeared on a wall in Londonderry late Sunday, claiming it was responsible for Wednesday’s attack on Detective Chief Inspector John Caldwell.

Two masked men shot Caldwell in front of his young son after the off-duty officer coached a children’s soccer team in Omagh, about 60 miles (nearly 100 kilometers) west of Belfast. Caldwell remained in critical condition in hospital.

Assistant Chief Constable Mark McEwan said Monday police were aware of the claim of responsibility and was “reviewing its contents as part of the overall investigation.”

Police said last week they were treating the attempted murder of Caldwell as terrorism-related, and that the New IRA was its primary line of enquiry.

Six men were detained for questioning.

Paramilitary groups in Northern Ireland put down their arms after the 1998 Good Friday peace accord largely ended three decades of violent conflict, known as “the Troubles,” between Irish republican and British loyalist groups and U.K. security forces.

But small IRA splinter groups have continued to launch sporadic attacks on security forces. The last fatal attack on a police officer in Northern Ireland was the April 2011 killing of Constable Ronan Kerr, who died when a booby-trap bomb exploded under his car in Omagh.