A shooter opened fire at a train depot in the Californian city of San Jose, killing eight people.
A police spokesman said on Wednesday that the alleged perpetrator was also dead.
Several people were injured in the incident.
The alleged shooter was a man.
The authorities did not initially disclose how he died.
The investigators could not say anything about the motive of the shooter either.
Police spokesman Russell Davis said the suspect was an employee of the company.
The train depot for light rail vehicles belongs to the local transport association VTA.
Several explosives were found at the crime scene.
In addition, there was a fire in the suspect's home early in the morning.
The investigation was ongoing, Davis said.
San Jose's Mayor Sam Liccardo spoke on Wednesday of a "very dark moment for our city and our community".
The first thing to do is to help those affected by the attack.
After that, he will focus his efforts on ensuring "that we never see a tragedy as terrible as this in San Jose again."
VTA boss Glenn Hendricks said: "A terrible tragedy happened today." Light rail vehicles are being serviced and dispatched in the affected facility.
The attack did not take place in the operations center.
The light rail traffic would be suspended, but buses would increasingly be used.
Firearms are widespread in the USA, and in many places the rules are very lax compared to other countries.
The CDC health authority recorded a total of 39,707 firearms deaths in 2019 alone - almost 110 per day.
Around 60 percent of the cases were suicides.