Secret Group Bought the Ingredients for a Dirty Bomb

…secret group of fewer than 10 people — formed in April 2014 in North Dakota, Texas and Michigan — discovered that getting a license and then ordering enough materials to make a dirty bomb was strikingly simple in one of their three tries. Sellers were preparing shipments that together were enough to poison a city center when the operation was shut down.

The team’s members could have been anyone — a terrorist outfit, emissaries of a rival government, domestic extremists. In fact, they were undercover bureaucrats with the investigative arm of Congress.…