The United States has reportedly not come to a definite conclusion that North Korea successfully conducted a nuclear test on Monday.

Foreign media outlets quoted a U.S. official as saying that based on the results of initial analysis, Washington has not found anything that confirms the North detonated a nuclear device.

U.S. broadcaster Fox News reported that while flying over the Korean Peninsula, the WC-135 “Command Phoenix” Air Force jet collected a second air sample to confirm the North’s nuclear test.

The first sample is being tested at a lab in New Jersey, while the second will be sent to Los Alamos, New Mexico, for further evaluation.

Fox said the U.S. has also been able to collect ground samples from particles blown by winds into the atmosphere. These particles will reportedly undergo testing as well.