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

The 28-year-old English midfielder looked to be on his way to Pittodrie in January 2016, but the deal fell through as the clubs fell out over a fee.
But he signed a pre-contract in March to join the Dons on a three-year deal.
"I always wanted to come here and I'm just delighted it has eventually been done," he told the Dons website.
"What's happened in the past has happened in the past, but it's been a long wait."
Tansey had returned to the Caledonian Stadium for a second spell at the club after leaving Stevenage in 2014.
He scored nine goals in 44 appearances this season but was unable to prevent Caley Thistle being relegated.
Tansey becomes the latest Caley Thistle player to have ended up at the other end of the A96.
Dons striker Adam Rooney and midfielders Jonny Hayes and Graeme Shinnie all moved to the Granite City having previously played for Inverness.
"I always had it in my mind to come here because of the set-up and from what the lads I know have said about the place," added Tansey. "It spoke volumes about the place.
"It was pretty much a no-brainer to come here.
"I've spoken to a few of the boys here - Graeme Shinnie, Jonny Hayes, Miles Storey, Adam Rooney and I've played against them over the years.
"They're all great lads and top players and it is exciting to play with players like that.
"That's what you want to do. It brings you on as a player and puts pressure on you to deliver for them as well as yourself."
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.
[NEXT_CONCEPT]
Greg Tansey is relieved to have ended his long wait to sign for Aberdeen after leaving relegated Inverness Caledonian Thistle.