Time of useful consciousness at 28,000 (last reply to ATC at FL277) is about 3 minutes, which should be enough time to get on a mask. Less than 3 minutes later the pilot failed to respond to ATC. It's a coinflip between medical event or pressurisation failure.
Was the oxygen cylinder open? If not, was it in reach of the pilot? We will in due course hear from the NTSB.