Gross misconduct notices have been served on two police officers who used a baton, a Taser and incapacitant spray on a man in a wheelchair.
Donald Burgess, who was also later handcuffed, died three weeks after the incident at a St Leonards care home.
The 93-year-old, who had dementia and one leg, was said to be threatening staff with a knife on 21 June.
A police watchdog probe is under way and both officers face a criminal investigation for manslaughter.
The Independent Office for Police Conduct (IOPC) said this did not necessarily mean disciplinary proceedings or criminal charges would follow.
Graham Beesley, IOPC regional director, said investigators would look at whether force used against Mr Burgess was "reasonable, necessary and proportionate in the circumstances" and whether it was in line with policy.
He said: "We have advised his next-of-kin of our independent investigation and explained the steps we will be taking."
The IOPC has not revealed which care home it was.
The officers - both police constables - arrived to be shown by care home staff into a room where Mr Burgess was holding a knife, the IOPC said.
Officers briefly engaged with Mr Burgess before one used PAVA spray - which is like pepper spray - followed by a baton.
The second deployed a Taser before the knife was removed from Mr Burgess and he was handcuffed.
Mr Burgess was taken to hospital after the incident and stayed there until he died three weeks later on 13 July.
The incident was referred to the IOPC by Sussex Police amid allegations of excessive use of force.
Investigators have taken witness statements, reviewed body-worn video footage, and also attended the post-mortem.
Mr Beesley said the cause of death was not yet known and the watchdog was awaiting the findings of further cardio-pathology examinations.
Sussex Police said the incident took place in the room where Mr Burgess lived.
Assistant Chief Constable Tanya Jones said: "We are providing every assistance to the IOPC to allow for a thorough investigation and for the family to receive the answers they need."
She said Sussex Police could not provide any further details as the investigation remained ongoing.