Armstrong released a statement following Green Day's performance Friday at Madrid's Mad Cool Festival.
The band took to the stage after festival goers witnessed an acrobat plunging to his death during a stunt.
Armstrong said his group was unaware of the tragedy when they continued on with the show.
"We didn't even know there was an acrobat performance at all," Armstrong wrote in the statement published to the band's official site on Sunday
. "These festivals are huge. There are so many things happening at the same time it's impossible to keep up with every performer/artist."
According to the BBC
, Pedro Aunión Monroy plummeted 100 feet from a cage while attempting a stunt.
Armstrong said he and other Green Day members "were in a back stage compound about a half mile away from the main festival stage" when their management was told there would be a delay because of a security issue.
The band was then transported to the main stage where they performed, he said.
"We were on stage at around midnight and played around two and a half hours," Armstrong said. "Everything seemed normal. The crowd and fans had a good time. We got off stage and drove back to our artist compound."
He said they were told of Monroy's death after Green Day's performance ended.
"If we had known prior to our performance we most likely would not have played at all," Armstrong said. "We are not heartless people. The safety and well being at any of our concerts absolutely comes first."
"What happened to Pedro is unthinkable," he added. "Once again we are heartbroken for his friends and family. And we are also shocked and heartbroken for anyone that had to witness this tragedy."