Agriculture Secretary Tom Vilsack, Iowa's former governor, called it "a very serious situation that requires immediate and decisive action."
He told The Associated Press by telephone Wednesday afternoon that there is no evidence the alleged activity interfered with the laboratories' work or results. But Vilsack said "we want to send a clear signal" that such actions won't be tolerated.
The names of the employees placed on leave weren't released. Officials said more could be involved.
Three USDA laboratories employ about 900 people on a sprawling campus in Ames. Testing for diseases such as mad cow and bird flu take place at the labs.
Cindy Smith, USDA's acting deputy undersecretary of marketing and regulatory programs, said in a telephone interview that the drugs were sold to other employees for their own and their families' use. The drugs were primarily antibiotics, blood pressure medications and pain relievers; none were narcotics.
"They are not concerned that they were being sold to anyone else, but there was inappropriate use being alleged of the veterinary credentials that allowed employees to gain access to medicines" that should have been purchased through a medical doctor, Smith said.
Vilsack said the USDA has commissioned two outside reviews of laboratory processes and management at the facilities. He said that results and possible corrective actions will be released once the investigation is finished.
I can't give you a time frame for the investigation. It's ongoing," he said. "It will be fair and complete."
Of the 19 employees placed on leave, all but two work for the USDA's Animal and Plant Health Inspection Service, which oversees two of the three labs at the site. That agency's acting administrator, Kevin Shea, reaffirmed that the alleged activities haven't affected the laboratories' work.
"We want to stress that, while we are extremely concerned with the conduct demonstrated at Ames, there is no evidence any test results or other official laboratory activities were compromised in any way or caused any animal health risk," he said in a statement.
The third Ames lab is operated by the Agricultural Research Service.
Officials refused to release other details, citing the ongoing investigation