Collinssplatt, who spent decades teaching English in Italy, lost his leg in a road accident when he was a young man and his arm in a horse-riding incident.
Police found traces of blood in the spare room of his apartment as well as in the lift that links the flat with the ground floor.
If Collinssplatt was attacked in the apartment, his assailant or assailants would have had to carry his body down to the ground floor and place it in the boot of the white Nissan Micra. The vehicle's engine was running when the body was discovered.
A post-mortem established that there were traces of carbon monoxide in the pensioner's blood, indicating that he was still breathing when he was in the boot of the car.
"There were no signs of a struggle," police said in a statement. "The cut marks are compatible with the use of a razor blade that was found on a windowsill in Mr Collinssplatt's apartment."
Tim Keates, an Oxford-educated English teacher and the victim's best friend for more than 35 years, told the Telegraph: "There are a few mysterious elements which suggest suicide, such as little cuts to the throat - people thinking of committing suicide sometimes do a 'dry run' beforehand.
"On the other hand, if it was suicide it seems rather a performance - he would have had to go from his first-floor flat, down to the garage, open it up, and get into the car. And why the boot, why not sit in the passenger seat?
Keates said he could think of no reason for anyone to want to murder his friend. "He was always jovial, optimistic and affable," he said.
"I never saw him depressed, not once. I saw him just a few days before his death and he was talking of organising a barbecue. I can't imagine anyone wanting to kill him. He was a reserved man but I never detected a breath of enmity or a grudge."
There was no sign of forced entry in the pensioner's apartment.
"He did use to leave the door ajar for his dog to come in and out," said Keates. "But he had nothing very valuable to steal."
Collinssplatt worked for years as the manager of an English language college in Modena, as well as for a local military academy and the University of Modena. He retired in 1993.
He lived with an Italian woman for a while but split from her a few years ago. He had no children.
Police are scrutinising his bank accounts and checking who he was in contact with in the days before his death on March 11.