Federal regulators said Friday they are launching an investigation into the improper shipment last week of nuclear material from Los Alamos National Laboratory to other federal labs around the country.

The National Nuclear Security Administration confirmed in a statement that it was informed by the New Mexico lab that procedures weren’t followed when shipping what was only described as small amounts of “special nuclear material” to facilities in California and South Carolina.

The material had been packaged for ground transport. But instead it was shipped via an air cargo service, which isn’t allowed by federal regulations.

It marks the latest gaffe by Los Alamos, the lab that created the atomic bomb. Criticism has been intensifying over the lab’s history of safety lapses as work ramps up to produce key components for the nation’s nuclear weapons cache.

“This failure to follow established procedures is absolutely unacceptable,” Frank Klotz, head of the National Nuclear Security Administration, said in a statement.

The agency oversees the lab along with other facilities that make up the U.S. nuclear complex. Klotz said it’s required that contractors who manage the labs, production plants and waste repositories rigorously adhere to what he called the highest safety and security standards as part of their national security work.

Once the investigation of the shipments to California and South Carolina is complete, the federal agency said any responsible parties will be held accountable.

Lab officials on declined to comment and referred questions to the National Nuclear Safety Administration.

Home to some of the nation’s top nuclear scientists and other researchers, Los Alamos has struggled for years to address management and oversight issues along with more recent safety concerns about the handling of radioactive waste and plutonium.

A series published this week by the Center for Public Integrity cited numerous internal reports and other documents outlining federal regulators’ concerns about safety lapses at the lab over the years, including a plutonium spill last summer and workers in 2011 positioning plutonium rods in a way that could have been disastrous.

In 2014, a chemical reaction stemming from Los Alamos inappropriately packaging a barrel of radioactive waste caused a radiation leak at the government’s only underground nuclear waste repository.

That misstep resulted in costly recovery work and a backlog in the multibillion-dollar program for cleaning up waste from decades of research and bomb-making.

There have also been reports that Los Alamos failed more than once in recent months to accurately document and label hazardous liquid that was shipped to a disposal facility in Colorado.

Klotz noted earlier this week that safety is paramount and that his agency withheld more than $82 million in contractor payments over safety and operational issues at the lab between 2013 and 2016.

The current $2.2 billion contract for Los Alamos National Security LLC to manage the lab ends in 2018. Some critics have said putting the contract out to bid will offer an opportunity to make changes at Los Alamos.

Greg Mello, with the Los Alamos Study Group, said the concern with the latest incident is that pressure changes could have compromised the packaging of the nuclear material once it was aboard the cargo plane.

“It’s like a cheap ball point pen in your shirt pocket,” he said. “It turned out fine this time, but the deeper problem is why are there so many kinds of errors.”

(AP)