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Lock had played a moderating influence from the beginning, telling his captors: "Look, if you start shooting people, if you kill even one of us, you know you will have no chance after that, don't you?" Lock wore his policeman's tunic throughout the siege, and even put on his hat when he spoke to his superiors through the embassy windows. The reason for Lock's formality: his .38-cal. revolver was hidden beneath the tunic. Lock told the other two British captives about the gun, but felt that if he tried to use it, he could not hope to shoot more than two of the gunmen before they got him, and the action would endanger the lives of all the hostages.
Monday the mood of the terrorists changed sharply for the worse. They announced that they would kill one hostage every 45 minutes until their demands had been met. Having successfully dramatized their cause to the world, they now seemed mainly intent on securing safe conduct for themselves to a friendly country—a concession that the British government had decided not to grant them under any circumstances.
The government was determined, Home Secretary William Whitelaw said later, not to allow "terrorist blackmail to succeed." Whitelaw, who served as Secretary of State for Northern Ireland in the early 1970s, was in charge of the government's handling of the crisis from the beginning.
Shortly before 7 p.m., the terrorists pushed the body of a dead hostage out the door of the embassy. It was Lavasani; he had volunteered, to be the first victim. By murdering a hostage, the terrorists apparently thought they could force the British government to meet their safe conduct demand. Instead, at Whitelaw's command, the killing triggered "Operation Nimrod," for which the S.A.S. force had been preparing for several days. Within 30 minutes, some 20 S.A.S. commandos, clad in black and wearing hoods, gas masks and armored vests, attacked the embassy from the roof and from adjoining town houses. They carried submachine guns, pistols and stun grenades, whose "thunderflash" blinds and deafens its victims for several seconds. Some slithered down ropes from the roof and threw grenades through the back windows. Then they leaped in after the explosions; others made a similar assault from the front balcony.
The explosion that blew out the front windows of the embassy was apparently caused by one or more stun grenades, thrown by a commando. The interior of the embassy was quickly reduced to near rubble as it caught on fire from the explosions. According to some reports, S.A.S. men also broke through a brick wall from an adjoining building when the assault began.
Inside the embassy, meanwhile, Hostage Morris said later, "there was smoke, screaming, explosions and gunfire." Bursting into the room where most of the male hostages were kept, the gunmen opened fire on their Iranian prisoners, killing one of them and wounding two others, including Chargé d'Affaires Gholam Ali Afrouz. Given the amount of terrorist fire occurring at that moment, said Morris, it was a miracle that only one man was killed. When one gunman took careful aim at an S.A.S. commando entering through a window, Constable Lock tackled the terrorist, and the commando shot the gunman dead.