A worker drives an electric cart past air monitoring equipment inside a storage room of the Waste Isolation Pilot Plant in Carlsbad, N.M. , shown in this undated photo. Handout/AP

Four days after a radiation alert shut the nation’s only underground nuclear waste facility, an independent monitoring center said Wednesday it found radioactive isotopes in an air sensor about a half mile from the southeastern New Mexico plant.

A filter from a monitor northwest of the Waste Isolation Pilot Plant (WIPP) in Carlsbad had trace amounts of plutonium and americium, said Russell Hardy, director of the Carlsbad Environmental Monitoring and Research Center.

“It’s still below what EPA considers actionable levels, but it’s important to know that some material did get out of the facility,” Hardy told the Albuquerque Journal.

The levels are the highest ever detected at or around the site, he added.

A WIPP air monitor detected airborne radiation underground late Friday night, setting off an alert, the Journal reported. WIPP reported the next day that its ventilation system had immediately switched to filtration mode, minimizing any potential release of radiation.

WIPP said in a statement Wednesday that its filters remove at least 99.97 percent of contaminants from the air, “meaning a minute amount still can pass through.”

“There is a lot more that needs to be known,” Don Hancock, director of the Nuclear Waste Safety Program at the Southwest Research and Information Center in Albuquerque, told local media.

“The big problem is, does anybody really know what happened in the underground and how much was released or is continuing to be released? And, therefore, how much is being captured by the filters and how much is getting into the environment?”

WIPP said Wednesday that it is developing a plan to safely re-enter the underground facility and that radiological professionals from other Energy Department locations and national laboratories will assist in the recovery.

Department of Energy (DOE) officials say most operations remain closed, but they have not released any further information.

In early February, a truck hauling salt in an underground mine at the site caught fire, shuttering operations for a few days. Officials said that fire was in an area separate from where nuclear waste is stored. In both instances, the DOE has said public safety has not been threatened.

Hardy said his center, an arm of New Mexico State University that monitors air, ground, and water samples from in and around WIPP, didn’t get the filters from the underground radiation sensor that was activated Saturday until Tuesday. He said he expects to have a reading from air sampling station closer to the plant next week.

He also noted that a second air sampling station 11 miles from the plant showed no radioactive particles.

WIPP is the nation’s first and only deep geological nuclear facility. It takes plutonium-contaminated waste from Los Alamos National Laboratory, and other defense projects, and buries it in rooms cut from underground salt beds.

Al Jazeera and wire services