PITTSBURGH — Federal antitrust authorities have asked Alcoa for further information about its offer to buy Canadian aluminum rival Alcan for $27.5 billion.

The Justice Department request was issued under the Hart-Scott-Rodino antitrust act and effectively extends a waiting period until 30 days after Alcoa fulfills the request.

The proposed deal also faces review by antitrust authorities in Canada, the European Union, Australia and Brazil, as well as foreign investment clearance in Canada, France and Australia.