Detectives went to a Diamond Bar home east of Los Angeles on Monday afternoon to investigate the disappearance of Brian Miller, 8, and twin brothers Evan and Christian, both 6.
"This is known as a child abduction. They're with their mother," sheriff's spokesman Steve Whitmore said.
"She could be in Nevada, she could be in Utah, we don't know," he said. "We are spreading a wide net."
Miller's spokesman, Scott Toussaint, did not immediately respond to phone and e-mail messages seeking comment on Tuesday morning.
The children were not believed to be in immediate danger. No Amber alert was issued, Whitmore said.
The mother was identified as Jennifer Dejongh, who also uses the name Jennifer Lopez. The children's parents are divorced and Dejongh does not have legal custody.
"She just wants to take care of her children properly. She's a good mom," her father, Jude Lopez, told Fox News.
Miller, 59, was first elected in 1998 to represent the 42nd Congressional District, which includes the communities of Diamond Bar, Brea, Chino and Chino Hills. The Republican lawmaker has three sons and a daughter.