A series of indiscreet emails by a Florida police chief has revealed his belief that Trayvon Martin's killer George Zimmerman is a volatile character who is "a Sandy Hook waiting to happen".

Steve Bracknell is the chief of the Lake Mary police department, which is investigating Zimmerman, 29, for allegedly threatening his estranged wife, Shellie, and his father-in-law with a gun on Monday.

In an email exchange with a Lake Mary resident critical of the department's initial decision not to arrest or charge Zimmerman, Bracknell appears to agree with the writer's description of Zimmerman, acquitted in July of murdering Martin, an unarmed black teenager, as "a ticking time bomb".

"Zimmerman is a Sandy Hook, Aurora waiting to happen," the resident, Santiago Rodriguez, told Bracknell in his first email, seeking an explanation for why Lake Mary officers did not charge Zimmerman.

"Your reference to Sandy Hook … I agree," Bracknell replied.

At the end of a follow-up email urging Bracknell not to "give up" on the investigation, Rodriguez wrote: "Sooner or later another mother and father is going to be on CNN lashing out against the system due to this man snapping."

Bracknell replied: "As for your final thoughts, again, on a personal note, I agree."

Neither the police chief, nor his department's spokesman, Zach Hudson, responded to a request from the Guardian for comment Friday, but the political blog ThinkProgress, which published the email exchange on Thursday night, said Bracknell confirmed the messages were authentic.

It followed an off-the-cuff comment Bracknell made to a reporter from Los Angeles on Monday in response to the massive media interest in the confrontation between George and Shellie Zimmerman at her father's house in Lake Mary.

"Man, it would be fantastic if you had an apartment out there [in California] for George Zimmerman. This guy is killing me," he said.

And in one of the emails to Rodriguez, he insists: "Rest assured, the last thing on planet earth I want is any relationship with the Zimmermans. Period."

Police said on Wednesday that no decision would be made for many weeks if Zimmerman should face charges over the incident, in which Shellie Zimmerman called 911 to report he was threatening her and her father with a gun.

"He's just threatening all of us with his firearm, and he's gonna shoot us. He punched my dad in the nose. My dad has a mark on his face," she said in the recording of the call, made as the couple argued over their belongings.

Shellie Zimmerman filed for divorce last week and, according to the police report, had gone to the house they once shared to pick up some of her possessions when her husband also turned up.

At some point during their confrontation, Shellie Zimmerman began recording him on an iPad, which he then allegedly smashed on the floor and cut with a pocket knife. Detectives say it could take months for forensic investigators to establish if any of the footage is retrievable.

The investigation stalled when Shellie Zimmerman, after talking with her lawyer, Kelly Sims, and her husband's attorney, Mark O'Mara, decided she did not want to press charges. She and her father, David Dean, then told police they had not seen George Zimmerman with a weapon, apparently contradicting what she told police dispatchers.

"I do not know why she changed her story. I could only guess, and that would be improper," Bracknell said in his email.

However, Shellie Zimmerman is serving 12 months' probation for perjury stemming from a lie she told a judge about the couple's finances at her husband's bail hearing last year. George Zimmerman told police that she hit him on the back with the iPad and that he smashed in to stop her.

Her backtracking left detectives relying on the iPad, which Hudson said was "in really bad shape."

"At this point, we do not have the tools available to effectively look at the video. As it stands right now, there will not be any charges anytime soon without that iPad," he said.

Bracknell described Monday's incident as "a very unusual occurrence" but denied there was any conspiracy or cover-up into why Zimmerman was released without charge after only a brief period "in investigative custody".

"The 911 tapes needs [sic] to be supported by physical evidence of the alleged physical violence. Since Shellie and her father refused to prosecute, we were powerless to arrest anyone," he wrote.

"Remember, we were the agency that wrote him a $256.00 [speeding] citation last week! No freebies here in Lake Mary."