 Nickell finally consented to a polygraph examination in November 1986 . She failed it and investigators narrowed their focus to her even further ; however , concrete evidence proving that Nickell had ever purchased or used cyanide was lacking , and despite their relative certainty that Stella Nickell had orchestrated the poisonings as either an elaborate cover @-@ up for an insurance @-@ motivated murder of her husband , or as a desperate attempt to force her husband 's death to be ruled an accident , to increase her insurance payout , they were unable to build a strong enough case to support an arrest . 
