Good Cop Bad Cop / Simon Kernick.

It's 14 years after the worst terrorist atrocity in UK history and, with the perpetrators either dead or missing, the motive remains a mystery. But Dr Ralph Teller, a wealthy industrialist who lost his wife in the attack, never gave up hope of justice. He thinks he knows the real identity of the mastermind responsible. Only one person can help him put together the final pieces of the puzzle - retired police officer Chris Sketty, who infiltrated the terrorist group and was wounded trying to prevent the attack. But Cleverly thinks Sketty is a liar...and possibly far worse He has a dossier on all the former cop's dark secrets and will make them public unless Sketty tells the truth. So, over one night, Sketty will share his brutal tale of betrayal, ruthlessness and corruption, finishing with a revelation so terrifying and unexpected that it will change everything. Is Sketty a brave hero, crippled in the line of duty, or the most ruthless mass murderer the country has ever seen?