The investigation entered its fourth day on Thursday, as FBI teams continued examining what authorities describe as a homemade science lab involving mixed chemicals.
FBI hazmat team swarms Irvine home in gated community
The incident began on Monday after a landlord reported suspicious circumstances at a home on Cartwheel within the Altair community.
The Orange County Fire Authority initially responded but found items concerning enough to call in Irvine police, who then requested assistance from the FBI.
Irvine police said a juvenile living at the home had mixed unknown chemicals, prompting the extended response.
The substances found inside are now being "carefully and methodically evaluated by subject matter experts," according to the department.
For days, FBI evidence response personnel - some in full hazmat gear - have been seen working inside the home's garage and under tents set up outside.
The agency said both its Evidence Response Team and Hazardous Evidence Response Team are involved in the effort.
Despite the heavy federal presence, police emphasized there is no known threat to the public as the investigation continues.