LOS ANGELES (Reuters) - Police in the San Francisco Bay area were seeking clues on Tuesday as to how a plastic bag of methamphetamine turned up in the Halloween candy of an 8-year-old girl.

The girl's father found a pinkish-colored Ziploc pouch containing a powdery substance while checking his daughter's trick-or-treat collection on Monday morning and called authorities, police said.

The father told police the bag apparently ended up in the child's Halloween candy while they were out trick-or-treating on Friday night near their home in the East Bay town of Hercules, California, about 25 miles northeast of San Francisco.

Testing of the substance, measuring one-tenth of a gram, revealed it to be crystal methamphetamine, "a small amount but enough to do damage to an 8-year-old girl had she gotten into it on her own," said detective Connie Van Putten, a spokeswoman for the Hercules Police Department.

She said detectives were re-interviewing the father and daughter on Tuesday to determine precisely which homes they visited on Halloween and were trying to see if fingerprints could be lifted from the plastic bag that carried the meth.

The father told police he and his daughter were trick-or-treating in a well-lit, 10-block area of town called the Promenade, consisting of quaint Victorian homes with picket fences, which is a popular gathering spot on Halloween.

Van Putten said the case appears to be an isolated incident, and detectives have no immediate evidence that the girl was singled out as a target.

"We are leaning toward it being an accident, but we are not ruling out an intentional act," she said. "Whoever it was may have intended to put it in somebody's candy but not hers."

A tenth of a gram of meth, while a relative small amount, is enough to bring felony charges for possession of a controlled substance, she said.

Van Putten said the case is a reminder for parents to look after their children's Halloween activities.

"You don't want to keep your children in bubble wrap, but you do need to be aware ... and go through children's candy to make sure none of the packaging has been tampered with, and that there's no needles, razor blades or anything foreign," she said.

(Reporting by Steve Gorman; Editing by Mohammad Zargham)