Nippon Steel made an aggressive all-cash offer for United States Steel that propelled the Japanese company into the lead in the final hours of a bidding war for the iconic American steelmaker.

Nippon Steel executives submitted a sweetened cash offer to acquire the Pittsburgh-based company for $55 (¥8,120) a share on Dec. 16, shortly after U.S. Steel’s bankers asked the suitor for a final proposal. U.S. Steel determined that weekend in a special meeting that Nippon Steel’s offer was superior to one from "Company D” — which according to a person familiar with the process was a $54-a-share cash-and-stock deal from Cleveland-Cliffs.

Details of the final hours before U.S. Steel’s decision emerged Wednesday in a regulatory filing that outlined a strategic review process that began in March. The document showed that five serious parties vied to buy U.S. Steel — four individual companies and a consortium. Nippon Steel made a $48-a-share cash proposal on Dec. 15, only to boost its offer by almost 15% within 24 hours.