His girlfriend Bree Halowell also became ill.
Now we're learning the apartment complex had been warned much earlier about dangerously high carbon monoxide levels.
Sheila Thomason, a former maintenance supervisor at Sagecrest, told KBOI News she warned tenants and the property owners association about high levels of carbon monoxide months before the young Marine died.
In a 2011 e-mail to the Sagecrest Property Owners Association obtained by KBOI, Thomason warned faulty water heaters at 26 apartments were creating high gas levels.
And she attached a plumbers report, also obtained by KBOI News, indicating some Sagecrest apartments tested at "forty times higher than the maximum allowable concentration for continuous exposure to carbon monoxide."
Thomason says water heaters mentioned in her e-mail were replaced and carbon monoxide detectors provided to those tenants during that replacement process.
She says the apartment where Forbush died was not one of the units mentioned in the e-mail.
Forbush's family has filed a wrongful death lawsuit, and an attorney for the family says the fact there was a prior warning helps their case.
"One of the elements that we're trying to prove is the parties, management company, property owner, and owners association, acted recklessly," said attorney Eric Clark.
But a lawyer for the property owners says his client was unaware of any widespread water heater problems.
KBOI News could not reach the property owners association for comment.