A partially-declassified Defence Intelligence Agency (DIA) report, written in 2012, reveals evidence that the US government has been following a policy of supporting ISIS for a number of years.
The report [embedded below] details the US support for jihadists in Syria.
The former head of the DIA, Michael Flynn, says that the government followed a policy that aided ISIS in its rise to power, and that the US pushed ISIS into Syria in order to destabilise the country.
Here is the redacted report:
Defence Intelligence Agency Report on ISIS
Here is a discussion with former DIA director Gen. Michael Flynn on the subject: