Adam Reechard Crespo, 43, has been charged with murdering his girlfriend Silvia Galva in July.
Ms Galva, 32, was impaled by a spear-tipped bed post after the couple allegedly got into a fight at their home in the city of Hallandale Beach in southern Florida.
Florida police think the couple’s Amazon Echo smart speakers could contain critical information about the alleged argument between the couple and have acquired audio recordings from two such gadgets.
There was a 12-inch double-sided metal blade spear at the foot of the bed and Ms Galva clutched the shaft as her boyfriend attempted to pull her off the bed, according to a police report seen by the South Florida Sun Sentinel.
Mr Crespo, who has pleaded not guilty to second-degree murder, is said to have then heard a snap.
According to an affidavit: “While he was still pulling her from the bed he heard a snap. The defendant turned around and discovered... the blade had penetrated the victim’s chest.”
Mr Crespo told police he pulled the blade out of his girlfriend’s chest “hoping it was not too bad” before telling a friend at the property to ring 911.
He insists his partner’s death was a tragic accident.
He told police he and his partner got into an argument in the bedroom of their home after they were drinking.
Hallandale Beach Police Department spokesperson Sergeant Pedro Abut told the Sun Sentinel they have got hold of the audio footage and are “in the process of analysing the information that was sent to us”.
Mr Crespo has been released on bail on a $65,000 (£50,000) bond.
A spokesperson for Amazon told the publication the smart speakers only work if people say the word “Alexa” or a “wake” word they have selected and the in-home listening devices do not normally record whole conversations.