A Brazilian woman suspected of poisoning her husband's family with an arsenic-laced Christmas cake has been found dead in her prison cell in what police think was most likely a suicide.

Deise Moura dos Anjos, 42, had been in pre-trial detention since January after prosecutors accused her of the murder on Christmas Eve of three relatives and the attempted murder of another three.

The victims had all eaten from the cake, which forensic experts found had been baked with flour contaminated with the deadly poison.

Police were also investigating if she may have killed her father-in-law, who died in September.

Forensic experts who exhumed his body found high levels of arsenic, which led them to believe that he, too, had been poisoned.

Moura dos Anjos denied any wrongdoing but local police chief Cléber dos Santos Lima told reporters last month that he was "certain that she researched, bought (...) and used the poison to kill her victims".

He added that police had found evidence showing that she had bought arsenic on four separate occasions.

Investigators tested many food items in the home of Moura dos Anjos's mother-in-law, where the six victims were taken ill, in order to find the source of the poisoning.

Eventually, they found sky-high levels of arsenic in the flour - some of which Moura dos Anjos's mother-in-law, Zeli dos Anjos, had used to bake the Christmas cake.

As Zeli dos Anjos had herself eaten from the cake and was taken seriously ill, police quickly ruled her out as a suspect even though she had prepared the cake.

Zeli dos Anjos survived but two of her sisters and one of her nieces died.

Her 10-year-old grandson and the husband of one of her sisters were also among those poisoned but recovered.