Monday's massive blackout in the Tokyo area was caused by workers who were trying to increase efficiency by erecting the crane that hit power lines while its barge was still moving, according to police sources.

Police were looking Tuesday into whether the operator of the barge, Mikuniya Co., a construction company based in Kamisu, Ibaraki Prefecture, is guilty of violating the electricity enterprises law by disrupting electrical transmission, the sources said.

The 43-year-old manager of Mikuniya's Chiba office and other company officials told police the workers erected the crane while the barge was still moving to a dredge site so they could get a head start.

The officials said they were aware of the Tokyo Electric Power Co. cables, contradicting earlier statements claiming they didn't know the cables were there.