A jury acquitted one man but could not reach a verdict on another in the deaths of 36 party-goers at a warehouse fire in Oakland, California.
Building manager Max Harris, 29, was acquitted of involuntary manslaughter over the blaze at the Ghost Ship venue on 3 December 2016.
A mistrial was declared in the same charges against his co-accused, leaseholder Derek Almena, 49.
The outcome follows a three-month trial and 10 days of jury deliberations.
Prosecutors alleged that Mr Harris, who considered himself the venue's creative director, and Mr Almena had flouted safety rules, allowing the largely wooden building to become a fire trap.
But the defendants' lawyers said the pair were not responsible, arguing that the fire was arson, though no official cause was ever determined by investigators.
Mr Harris and Mr Almena had each faced 36 counts of involuntary manslaughter, carrying up to 39 years in prison if they were convicted.
Alameda County Superior Court Judge Trina Thompson on Thursday gave prosecutors until next month to decide whether Almena will be retried.
Jurors were 10-2 in favour of convicting Almena but could not reach a unanimous verdict, the East Bay Times reports.
The 10,000 sq ft (940 sq m) warehouse, which had been converted into a space for artists to live and to work, was hosting an electronic dance concert on the night the fast-moving blaze broke out.
The young revellers died of smoke inhalation after they became trapped on the second floor of the venue, which was packed with furniture, artwork, pianos and other flammable material.
The trial heard that the Ghost Ship had only two exits and no fire alarms, smoke detectors or sprinklers.
The co-owners of the building, Chor Ng and her two children, were not charged in the case.