Complex sentence: Fort Saginaw Mall had become a dead mall by the 1980s, as many of its stores had closed; by the end of the 1990s, the entire mall was closed off, except for an attached Kmart which remained open until May 2004.
Split into simpler sentences: Fort Saginaw Mall had become a dead mall by the 1980s, as many of its stores had closed. By the end of the 1990s, the entire mall was closed off, except for an attached Kmart which remained open until May 2004.