The U.S. Federal Aviation Administration said on Monday it is proposing a $350,000 civil fine against Inc. for shipping a product that allegedly violated hazardous materials regulations and injured several UPS workers who handled the package.

The penalty is the largest fine the FAA has proposed imposing on Amazon, which the agency said has had a series of at least 24 hazardous materials violations in recent years.

The FAA alleged that on Oct. 15, 2014, Amazon sent a UPS package with a 1-gallon container of "Amazing! LIQUID FIRE," a corrosive drain cleaner for transportation by air from Louisville, Kentucky, to Boulder, Colorado.

The package leaked and nine UPS employees who came into contact with the box reported feeling a burning sensation and were treated with a chemical wash, the FAA said.

The FAA said the shipment was improperly packaged, not accompanied by a declaration for dangerous goods and not properly labeled to indicate the hazardous contents.

Amazon declined to answer questions about the incident or prior fines, but said in a written statement that "we ship tens of millions of products every day and have developed sophisticated technologies to detect potential shipping hazards and use any defects as an opportunity for continuous improvement. We will continue to partner with the FAA in this area."

UPS spokesman Mike Mangeot in a statement declined to discuss the incident "other than to say our employees were fine after receiving treatment."

UPS is committed to the safe transportation of hazardous materials, he said. "We expect our customers to follow hazmat regulations, and when incidents or discrepancies occur, we report those situations," Mangeot said.

The FAA alleged Amazon failed to provide emergency response information with the package, and Amazon employees who handled the package had not received required hazardous materials training.

The FAA said that from February 2013 to September 2015, the government found Amazon had violated the hazardous materials regulations 24 other times seeking a total of nearly $1.3 million in fines. In at least 15 instances, hazardous material leaked, the FAA said in its notice of violation reviewed by Reuters.

It was not clear how many total times Amazon paid fines or admitted responsibility, since the FAA does not issue press releases for fines below $50,000, an FAA spokesman said. The company did pay $91,000 in April 2014 for a 2013 incident.

The agency is continuing to investigate Amazon’s compliance with hazardous shipment rules.

In April 2014, Amazon paid a $91,000 fine to the FAA after its employees improperly shipped a package in 2013 containing flammable liquid adhesive by air on Federal Express from Whitestown, Indiana, to Boulder, Colorado.