Scientists are analysing the ingredients in capsules sold as "generic Tamiflu" to online customers, Dave Elder, director of the Food and Drug Administration's office of enforcement said.
"First of all, there is no such thing as generic Tamiflu," Mr Elder said, noting that Swiss pharmaceutical company Roche is the only legitimate maker of the vaccine.
"There are people who are trying to profit from the heightened concerns about bird flu in this country."
It had yet to be determined exactly what ingredients were in the capsules sent in the 51 express mail packages seized by US customs agents at a San Francisco international mail facility since November 26.
Preliminary testing confirmed the capsules did not contain the active ingredient in Tamiflu.
"At this point, we haven't found anything harmful," Mr Elder said. "The harm comes from people believing they are being protected from the flu. They are not getting the benefit they expect."
Packages were addressed to a variety of people in the US and contained bottles of from 10 to 50 capsules.
"We have customs officers at the airmail facility to check whether things coming in are what they say they are," Roxanne Hercules of the US Customs and Border Protection Agency.
"We saw labels that said 'generic Tamiflu,' and there is no generic Tamiflu. Obviously, that is a big tipoff."
Packaging bore Chinese writing, prompting US investigators to seek the help of counterparts in Asia to track down the point of origin.
"We will follow any direction the investigation points us in," Mr Elder said.
The FDA sent letters a few weeks ago to companies that were hawking "snake oil," unproven and dubious concoctions promising to cure or prevent avian flu, according to Mr Elder.
"We anticipated people would do fakes and counterfeiting," Mr Elder said. "We are hoping it doesn't continue, but we are gong to stay on top of it to make sure it doesn't."
Mr Elder advised people to coordinate with their doctors and pharmacists to make certain mail-order medicines are legitimate.