SAN FRANCISCO - Google's lucrative online advertising system is facing a US government investigation that is expected to cost the Internet search leader at least $500 million.

The company disclosed the Justice Department probe in a regulatory filing Tuesday. The documents provided few details except that the investigation involved how some unnamed advertisers used Google's advertising network, which generated $8.3 billion in revenue during the first three months of the year.

Google says it decided earlier this month to set aside $500 million to cover a possible settlement. That move resulted in a charge that lowered the first-quarter earnings that Google announced in mid-April. With the change, Google's net income fell from the previously reported $2.3 billion, or $7.04 per share, to $1.8 billion, or $5.51 per share.