An American jihadist who set up the terrorist training camp where the leader of the 2005 London suicide bombers learned how to manufacture explosives, has been quietly released after serving only four and a half years of a possible 70-year sentence, a Guardian investigation has learned.

The unreported sentencing of Mohammed Junaid Babar to "time served" because of what a New York judge described as "exceptional co-operation" that began even before his arrest has raised questions over whether Babar was a US informer at the time he was helping to train the ringleader of the 7 July tube and bus bombings.