This is a silly unsourced scare-mongering article.
If you accept the article at face value, which you shouldn't, some simple math should make you go "what???"
It states that it takes 7 pounds of plutonium to make a bomb. Multiply that by the 800 times that Nagasaki can be bombed. That's 5600 pounds of plutonium. Or 2.8 tons. What about the other 3.2 tons that are part of the article's title?
A dirty bomb is only going to kill those that are close enough to be affected by the explosion of the non-nuclear bomb. It is a wonderful scare mechanism, but if a terrorist is looking to maximize carnage, it would be better to put his efforts and funding into making more of the non-nuclear bomb component.
Nuclear material was lost. Ok, here are some things a real article would have answered: How much is fissile? How much is cobalt? Cesium? How much is non-radioactive material that has some radioactive contamination?
Why were these agent in San Antonio allowed to transport plutonium by car? Was it legal?
Who might have the other 5.999 tons of radioactive material?