The incidence of sabotage and vandalism to the U.S. electric grid might be less severe than data reported to the federal government suggest, a Bloomberg BNA data analysis shows.

A high-profile attack on the Metcalf substation in San Jose, Calif., during an upswing in reported attacks to the Department of Energy in the last seven years led to new Federal Energy Regulatory Commission security regulations for electric grid facilities as well as various proposals in Congress to prevent future attacks.

But cases of sabotage and vandalism may have been over-reported to the Energy Department because the reporting requirement was unclear....