Uta Von Schwedler, 49, was found dead -- in the bathtub, at her Sugar House home -- in September 2011.

But despite being the prime suspect for more than a year-and-a-half, John Wall, 49, was not arrested until Thursday morning.

Authorities claim the victim drowned -- but a lethal amount of Xanax was also found in her system.

The couple's 19-year-old son says he believes his father killed his mother.

Wall is now being held -- in the Salt Lake County Jail -- on a $1.5-million, cash-only bail.