Six Baltimore police officers have been suspended over the death of a man whose neck was broken after he was arrested and locked in a police van, as it emerged officers had delayed providing him with medical attention despite his requests.

Freddie Gray died from a “significant spinal injury”, police confirmed on Monday, while claiming it remained unknown how he was hurt. Chiefs said Gray appeared to have been injured while locked alone in a compartment of their transportation wagon.

“When Mr Gray was put in that van, he could talk, he was upset. And when he was taken out of that van, he could not talk and he could not breathe,” deputy police commissioner Jerry Rodriguez said at a press conference.

“It’s clear that what happened happened inside the van,” said mayor Stephanie Rawlings-Blake. “We don’t have any procedure that would have an officer riding in the back of the van with the suspect.”

Rodriguez said all officers involved who had been interviewed had denied using force against Gray. A criminal inquiry has been opened into Gray’s death. City officials said the investigation would be completed by Friday 1 May and then handed to state prosecutors, who would decide whether or not to bring criminal charges.

Providing the most detailed timeline of events surrounding Gray’s arrest and detention on 12 April, senior city officials revealed he had asked officers for an asthma inhaler two minutes after he was apprehended, before he was placed inside the police van.

Yet the van made two stops before medical assistance was radioed for, the officials said. During the second stop, another prisoner from a separate incident was placed inside the van in a compartment walled off from Gray.

Rodriguez told reporters before this second stop, which occurred about 13 minutes after Gray was placed in the van, the driver had requested an additional unit to check on Gray, who had been placed in leg irons during the first stop after “acting irate in the back”.

The 25-year-old eventually received medical attention half an hour after the police van departed, according to the police timeline of events.

Gray died on Sunday, a week after being chased and arrested by officers at 8.39 am. His family say he lapsed into a coma after his spine was “80% severed” at his neck and his voice box was injured. Police said an autopsy was completed on Monday and confirmed the spinal injury led to Gray’s death.

During Monday’s press conference Rodriguez said Gray was arrested after a lieutenant had made “eye contact” with two individuals – one of whom was Gray – and then Gray fled on foot. Rawlings-Blake stressed that city officials appreciated that it was not an offence to “run away”.

A court filing from the day of Gray’s arrest showed he was charged with illegally carrying a knife, which an officer said was discovered inside one of Gray’s pockets. However Rawlings-Blake said: “We know that having a knife is not necessarily a crime, not necessarily a probable cause to search someone.”

Billy Murphy Jr, an attorney for Gray’s family, said earlier on Monday the knife charge did not explain why Gray was chased and searched.

“How did they have x-ray vision or any knowledge of that?” Murphy said. “If you arrest a man illegally the evidence you find afterwards is not admissible. There’s always the temptation – because you’re dealing with human beings – that they’re going to cover this up in a way that makes them look faultless.”

Rodriguez confirmed Gray “gave up without the use of force” and while one officer unholstered his Taser, he did not deploy it during the arrest. Footage recorded by a nearby city surveillance camera, which was played at Monday’s press conference, did not appear to shed light on what happened.

Cellphone video of Gray’s arrest released last week showed him being dragged into the police van by officers. While he was shouting in apparent pain and moving his head, at least one of his legs appeared limp. The video did not show his initial treatment by police.

Rawlings-Blake said there was no footage recorded inside the van. Police said the other prisoner in the back of the van had been interviewed. An “independent review board” is to be convened by police chiefs to look into the incident and report back on whether procedures were followed.

An account of the incident in the court filing, written by an officer Garrett Miller, said that after being “arrested without force or incident,” during his transportation to the police’s western district headquarters Gray “suffered a medical emergency and was immediately transported to Shock Trauma via medic.”

Rawlings-Blake, speaking alongside Baltimore police commissioner Anthony Batts, called for calm among city residents who have protested the death and called for an indictment of the officers responsible. “We are a community on edge,” said Batts.

“I understand the community’s frustration. I understand because I am frustrated,” Rawlings-Blake said. “I’m angry that we are here again, that we have had to tell another mother that her child is dead.”

However Rodriguez said: “This is not Ferguson,” referring to the Missouri town where unrest followed the fatal police shooting of an unarmed 18-year-old last August. “This is a city where we work very hard with the current administration to enter into dialogue with the community,” he said.