What building construction company built the John Hancock Tower in Boston, Massachusetts?

The actual building company is not given in readily-available sites, but the atchitects and supervisors of construction are. "The building known by Bostonians as the John Hancock Tower, or colloquially simply The Hancock, is officially named Hancock Place. It is a 60-story, 790-foot-tall (241 meter) skyscraper designed by I.M. Pei and Henry N. Cobb of the firm now known as Pei, Cobb and Freed and was completed in 1976."
https://en.wikipedia.org/wiki/John_Hancock_Tower

Gilbane Building Co. of Providence

By Aug. 28, 1968, one week after the ceremonial groundbreaking, it was time for the Gilbane Building Co. of Providence to transform Cobb's drawings into a glass-and-steel tower. It would not be easy.

http://www.boston.com/news/local/massachusetts/articles/2006/09/24/60_stories_and_countless_tales/?page=3

Legal action: The other defendants are Gilbane Building Company, Jeremiah Sullivan & Sons, Inc., Spencer, White & Prentis, Inc., Carter Pile-Driving Inc., Max Phillipson & Associates, Inc., I. M. Pei and Partners, and Mueser, Rutledge, Wentworth & Johnston.

http://masscases.com/cases/sjc/399/399mass43.html