Kyneton death: Lethal substance that killed suspect during police raid as five officers are rushed to hospital

Five officers were taken to hospital after a man in their custody died when he swallowed a highly lethal poison during a police raid of his home.

Police arrested the man, 27, at a Kyneton home in the Macedon Ranges, about 86km north-west of Melbourne on Tuesday.

They called for medical assistance and began CPR after the man became 'unwell and unresponsive'.

The man died at the scene, despite the efforts of emergency responders to revive him.

Five officers who were inside the home at the time weren't affected but were taken to hospital as a precaution.

A forensic analysis of the substance the man swallowed is ongoing.

Police sources have since claimed the man swallowed sodium cyanide, the Age reported .

The man's devastated family are now demanding answers over what happened during the arrest, Nine News reported.

Officers attended the home as part of an ongoing investigation, which a Victoria Police spokeswoman declined to comment on.

As the man died in police custody, the professional standards command and the homicide squad have launched investigations into the man's death.

Sodium cyanide is mainly used for industrial purposes, especially in the gold mining industry, and a 2010 government assessment revealed that it was also used in metal cleaning, research, and pest control.

The chemical has since been widely banned and is not registered by the Australian Pesticides and Veterinary Medicines Authority for agricultural or veterinary usage.

Crisis support is available from Lifeline on 13 11 14 and Beyond Blue on 1300 22 4636.