资料题：How did the police get the report（　）。

In the largest theft in British history, six masked gunmen stole almost $40 million in pure gold from a warehouse near Heathrow Airport.The robbery took place early this morning.Two boxed of diamonds were also taken, although it was not immediately known how much they were worth.According to the police, the robbers, who were carrying handguns and wearing what looked like ski masks, entered the unmarked warehouse at about 6:40 a.m.They handcuffed several guards who were working the night shift, poured gasoline on them, and said that they would setthem on fire if they did not cooperate.One hour later, a guard was able to get out of the handcuffs and ring the alarm.When the police arrived, there was no sign of the robbers.The police immediately closed the warehouse and began to investigate the theft.They said that they had no idea how the thieves were able to avoid the complicated security system, whichinclude automatic doors, a complex alarm system, light, and television cameras.They also didn't know how the men were able to remove the 6,800 bars of gold, which weighed about three tons.One police officer said,“They must have had at least two large trucks.'”The police did not say who owned the gold, which was supposed to have gone to the Far East.The insurance company for the warehouse has offered a $3 million reward for information about the robbery.