Three West Mercia Police officers had now been interviewed under criminal caution, the Independent Police Complaints Commission (IPCC) said.
Mr Atkinson died after contact with police in Meadow Close, Trench, Telford, on 15 August last year.
West Mercia Police said it continued to co-operate with the IPCC.
The three officers had been served with gross misconduct notices, the police watchdog said.
IPCC Commissioner Derrick Campbell said: "At the conclusion of our independent investigation I will consider whether to refer our investigation report to the Crown Prosecution Service to make a decision on any potential charges against police officers.
"A criminal investigation does not mean that criminal charges will necessarily follow."
Mr Campbell said its independent report would consider whether any officers should, in the watchdog's view, face misconduct proceedings.
Investigative inquiries had ended, but the IPCC investigation report could not be finalised "until the full pathology and toxicology reports are provided", the watchdog said.
It said IPCC investigators had spoken to about 15 other police officers as witnesses, along with more than 15 members of the public and paramedics.
Forensic experts had also been consulted.
A Taser used during the incident had been analysed and investigators had "transcribed relevant police Airwave radio transmissions and examined police policies on the use of force".
West Mercia Police Assistant Chief Constable Martin Evans said the force's "thoughts and sympathies" were with Mr Atkinson's family and friends and it continued to co-operate with the IPCC throughout its investigation.
He said: "It would not be appropriate for us to comment further at this time while that investigation is ongoing."
Mr Atkinson started his career at Ipswich Town before moving to Sheffield Wednesday, Real Sociedad, Villa and FenerbahÃ§e
A third police officer has been investigated over the death of ex-Aston Villa star Dalian Atkinson who was shot with a Taser, it has emerged.