From Sean Howe's Marvel Comics: The Untold Story:
Before [Mark] Gruenwald left for his weekend home on August 9, he grabbed a preview copy of Rob Liefeld's Captain America #1, It was Gruenwald's favorite Marvel character; until a few months earlier, he'd either written or edited every issue since 1982. On Monday morning, rumors strated flying around the offices, confirmed by an 11 a.m. email from Terry Stewart. "It's with my deepest and most profound regret that I inform you that Marl Gruenwald passed away unexpectedly early today at home," the note began. The cause of death was a heart attack.