I am trying to work this idea into a short story and decided to throw it out here

Terrorists in the United States built a very effective bomb out of fertilizer and diesel fuel. Large ships, called bulk carriers, carry, among other things, fertilizer. There are about 7000 of them in service worldwide. They are powered by diesel engines. For a bulk carrier to take on a load of fertilizer and then a large amount of diesel fuel would not raise eyebrows anywhere. If they went to another port for more fuel, who would notice?

Bulk carriers carry from 10,000 to 100,000 DWT [dead weight tons] of fertilizer. This is a LOT more than a Ryder Rental truck carries. We all saw what that Oklahoma City bomb did. An entire ship could be turned into one humungous bomb. These ships arrive at U.S. ports every day.

Would current procedures stop this kind of attack?.