BOSTON — An independent engineering consultant hired by NStar to investigate a transformer fire and power failure that knocked out power to some 22,000 customers in Boston’s Back Bay in March has been unable to pinpoint an exact cause.

Some customers were without power for days.

The 220-page report by RTI Group ruled out human error because no one entered or left the substation in the 18 hours before the fire.

A connection failed and sparked mineral oil, used as a cooling agent and conductor in the power line. But the root cause, the failure in a high-voltage connection, may never be solved.

An NStar spokesman tells The Boston Globe the utility will follow up on recommendations in the report.