OK!
A Liberian-registered ship, approaching
a NJ port, is scanned by a USCG radiation detector. A positive radiation reading is detected 'initially'.
The ship is forced to anchor 6-miles
off the NJ coast. And then SEALS board
the ship [a day or two later?] with their own detectors and they
claim "that no radiation was detected."
The Feds conclude that the initial readings were from 'natural' radiation
coming from slate-tiles carried by the freighter.
Huh??????

Something doesn't add up here!!

What can you conclude?
Here are some possible suggestions:

[1] The Feds are lying?

[2] The USCG in the NJ waters had a faulty radiation-detector?

[3] After the initial radiation detection, any radioactive contraband onboard was tossed into deep water overboard [at night?] by the crew?

[4] Cosmic rays?

[5] Or the ship really did have radioactive contraband onboard for sufficient time to induce temporary radioactivity in the slate-tiles cargo, prior to the ship reaching American waters...And before the USCG could approach to within visual or radar-range
of the ship [within 200 miles of the NJ coast] a Zodiac was used to take the radioactive contraband off the ship.

"You can fool some of the people
all of the time...You can fool most of the people part of the time...But you cannot fool all of the people all of the time": ABRAHAM LINCOLN.

Jacob