Attorney General Jerry Brown has filed a lawsuit against 20 U.S. toy companies and stores, alleging they knowingly manufactured and sold toys with unlawful amounts of lead.

The lawsuit names companies such as Mattel, Fisher-Price, Toys "R" Us, Wal-Mart and Costco, and says they did not provide enough warnings about risks associated with lead in toys.

According to the lawsuit, the companies violated the Safe Drinking Water and Toxic Enforcement Act of 1986, also known as Proposition 65, which was approved by voters. The law says businesses must provide "clear and reasonable warning" before exposing people to chemicals, such as lead, known to cause cancer and reproductive harm. The state can require companies to put warning labels on products or to adopt more stringent inspection procedures, according to Reuters News Service.

The suit comes on the heels of a number of highly publicized toy recalls over products that contained lead and other toxic substances. More than 6 million toys have been recalled this year because of lead.

If the state succeeds, the companies would have to label products with lead and be fined up to $2,500 per day for each violation.