Police officers and SWAT personnel surrounded a Fountain Valley home about 1 p.m. Tuesday.
A police helicopter and Department of Homeland Security personnel also were on the scene.
A man who said he lives in the house told the Daily Pilot that police told him and his family to leave but didn't give a reason.
Westminster police said they are conducting an investigation but would not provide information about it.
Fountain Valley police assisted Tuesday but also did not comment.
The residence, in the 9600 block of Talbert Avenue, is near Fountain Valley High School, which was placed on lockdown earlier Tuesday because of what authorities described as police activity near the campus.
Josh Lamar, Fountain Valley High's assistant principal, and the Huntington Beach Union High School District tweeted at about 12:45 p.m. that the school had been given the all-clear to resume normal activities, about 2½ hours after the lockdown began.
Still, Lamar said, "Talbert [Avenue] is not accessible for safety reasons. All traffic entering campus must leave heading northbound on Bushard [Street]."
Earlier, Lamar tweeted shortly after 10 a.m. that students and staff were safe, but he urged people to avoid the area near Talbert and Bushard. He also told parents not to call the school.
At about 11:30 a.m. Lamar tweeted: "We are being permitted to resume our bell schedule. We do not have the all-clear as of now, so students are not being released."
Some users of Huntington Beach Community Forum, a Facebook group, said they received automated calls from the school informing them of the lockdown.
3 p.m.: This article was updated with information about the Westminster Police Department.
1:50 p.m.: This article was updated with the news of the police action at the house and the lockdown being lifted at Fountain Valley High.
12:10 p.m.: This article was updated with a new tweet from Josh Lamar.