No word on the cause of death. However, a statement as been put out by singer/co-founder Donald Fagen.
http://www.rollingstone.com/music/news/walter-becker-steely-dan-co-founder-dead-at-67-w500956
http://www.rollingstone.com/music/news/walter-becker-steely-dan-co-founder-dead-at-67-w500956