The New York Times, citing "several lawyers briefed on the matter" who were speaking on the condition of anonymity, reported on Wednesday that Guillen, who was left off the Giants' roster in favor of Aaron Rowand, was the subject of a phone call from federal authorities, who informed MLB that they were looking into shipments of human growth hormone that were sent to Guillen's wife in the Bay Area.
The Times reported that the Commissioner's Office began its own investigation into the matter and advised the Giants to leave Guillen off the playoff roster. An MLB spokesman declined comment to the Times.
Guillen also was tied to PEDs in 2007, when the San Francisco Chronicle reported that the outfielder purchased more than $19,000 worth of HGH and other substances from a clinic in Florida from 2002-05.
The Times reported Wednesday that federal authorities, including Drug Enforcement Administration agents, are involved in the investigation.
Doug Miller is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.