Previously imprisoned London Bridge attacker was reportedly wearing a GPS police tag

Scotland Yard is trying to figure out how a man previously convicted on terrorism charges was able to carry out an attack in London despite wearing a GPS tracker.

A 28-year-old man who had previously been jailed for his role in a plot to blow up the London Stock Exchange in 2012 carried out a stabbing attack Friday at London Bridge that resulted in the deaths of a man and a woman Friday and injuries to three others. Police believe that Usman Khan, who is affiliated with an al-Qaida-inspired terrorist group, acted alone in the attack, which is considered a terrorist incident. Khan was shot dead by police after civilians restrained him.

Khan was reportedly wearing a GPS police tag and was out on parole, prompting investigators to attempt to determine how he was able to launch the attack despite authorities monitoring his movements. The Parole Board said it wasn't involved in Khan's release from prison and that he "appears" to have been automatically released as required by law. He was initially sentenced to an indefinite term with a minimum of eight years in 2012, but that was changed to a 16-year fixed-term sentence and an extended period on parole by the court of appeal in 2013. The judge who made the decision said there was "no doubt" Khan "could legitimately be considered dangerous."

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Police confirmed that, just prior to the attack, Khan was attending a Cambridge University conference on prisoner rehabilitation. Read more at BBC and The Guardian.