3 books which should be available at a local library titled: "Soldiers of the Great War", they list all U.S. Soldiers which lost thier lives, thier names are listed by state, how they died (KIA or of wounds), their rank at death then by name with home town if known.
A William Keefe from Pittsburg is listed in Volume 3, Pennsylvania, KIA and was a Sgt. Since he was KIA means he died oversea and I checked the American Battle Monuments Commision website that has the names of U.S. Service Personel buried in overseas cemeteries and didn't see him listed so his body must have been returned to Pennslyvania forburial. Check the localpapers of the time and most counties have memorials for WWI Veterans so the historical/genealogical society of Allegheny County might have some information. There was a fire at the NARA in 1973 only 80% of records were destroyed, maybe William's servic record was in the remainig 20%.