New York Times: Feb. 5, 1927:

Guy Lowell dies; noted architect.
Designer of New York's new county court house succumbs in Madeira.
Cousin of Harvard head.
Boston Museum of Fine Arts among his notable works - won Italian medal for valor in war.
Guy Lowell, architect of the new County Court House at Centre and Pearl Streets, which is to be opened on Feb. 17, died suddenly yesterday in Madeira, according to word received here late last night from his home in Brookline, Mass. The circumstances of his death were not disclosd. He was 56 years old.
A cousin of President Lowell of Harvard, and a son of the late Edward J. Lowell of Boston, who was a fist cousin of James Russell Lowell. Mr. Lowell was a member of one of the leading families of New England. He was born in Boston, graduated from Harvard in 1892, and after two years at the Massachusetts Institute of Technology went to the Beaux Arts in Paris, graduating there in 1899. While at Harvard Mr. Lowell was the intercollegiate champion in the mile run.
Returning to Boston, Mr. Lowell began the practice of his profession there in 1900. Since then he has designed many important buildings, several of them of great public interest, notably the Boston Museum of Fine Arts and the New York County Court House, already mentioned. The later structure is now about to be used after fourteen years of discussion and building. The design that won the award for Mr. Lowell in 1913 was a remarkable one, circular in form, and on that account, with alleged unsuitable attributes, it was subjected to much adverse criticism.
It was stoutly defended, however, and was highly praised by such authorities as the Architects and Builders' Journal of England as "one of the extraordinary achievements of recent American architecture, a conception of great power and directness. No public building of modern times seems to us to epitomize so finely the best characteristics of the age - power, law and order." But the criticism of Judges, the men who were destined to make use of the building, coupled with the war's effect on its cost, caused the original approval to be withdrawn. Mr. Lowell then submitted a design, hexagonal in form, the one finally adopted. The cost of the Court House to the taxpayers is estimated at $30,000,000.
During the World War Mr. Lowell served with the Red Cross in Italy with the rank of Major and the position of Director of Department of Military Affairs in Italy. His fearlessness under fire won him the Italian military medal for valor. Two years ago he was made a Vice President of the Italy-America Society. In many ways Mr. Lowell had given his time and labor for projects of public welfare. He was a trustee of Simmons College, sole trustee of the Lowell Observatory at Flagstaff, Ariz.; recalling that Percival Lowell, the astronomer, was his cousin.
Among the private residences he designed were those of C.J.G. Billings, Paul D. Cravath, Clarence H. Mackay and Harry Payne Whitney. He also designed gardens for the elder J. Pierpont Morgan, Andrew Carnegie and the Piping Rock Club. He was an ardent yachtsman. His boat, the Cima, was one of the three American entries in the Kiel regatta of 1911. He married Henrietta Sargent, daughter of Charles S. Sargent of Brookline, Mass. He had an apartment in this city at 22 East Fort-seventh Street, and offices here as well as in Boston. Among his many clubs were the Brook. New York Yacht and Piping Rock.

Boston Globe, March 9, 1927:

Funeral services for Guy Lowell, dinstinguished architect, who died Feb 4 at Madeira, were held yesterday afternoon at Emmanuel Church. Friends filled the church to capacity.
Dr. Elwood Worcester conducted simple servies, at which there were neither pallbearers nor ushers, and the full-vested choir sang. Men from Mr. Lowell's Boston and New York offices, some of them his associates for 26 years, carried the coffin. Burial was in Walnut Hill Cemetery, Brookline.

Lowell dies of a stroke (Life magazine, March 18, 1957, pg. 130) or cerebral hemmorage (death certificate)