Skripal poisoning suspects received mystery phone call following attack
Detectives think pair may have been awaiting signal that operation had been successful before returning home
The Russian men suspected of poisoning Sergei and Yulia Skripal in Salisbury received a phone call after returning to London on the day of the alleged attack, raising the possibility that a backup team played a role in the operation.
One theory being considered by investigators is whether the call, which has not been disclosed before, was a signal to tip them off that the operation had been a success.