St. Paul police said Friday that no criminal activity has been linked to an apartment fire that killed a woman and her young son.

Police spokesman Steve Linders said authorities conducted an “extensive and thorough” investigation and do not believe the Wednesday morning fire at 1035 Arkwright St. N. was criminal in origin.

Authorities have not said what might have caused the fire. A message left with the St. Paul Fire Department was not immediately returned.

Tiffany M. Alexander, 27, died in the blaze in the family’s apartment. Her son, William Mangan, 2, died the next day. Her nephew, Gabe Alexander, 3, and mother, Thereza “Tess” Alexander, 47, were badly injured. Family members said the two are expected to recover.

A GoFundMe page for Tiffany Alexander’s family has been set up at gofundme.com/helpalexanderfamily.