Puerto Rico has been "destroyed," and that is why it's a problem getting all the relief supplies to the island, said Michael Brown, the former FEMA head who was in charge when Hurricane Katrina struck in 2005.

Fuel and relief items such as food, medicine and water have been in short supply since Hurricane Maria struck Puerto Rico more than a week ago.

"This is truly a logistical nightmare for FEMA," Brown said in an interview with CNBC's "Power Lunch" on Friday.

With ports, railroads and airport runways damaged, a damage assessment must be done and repairs made.

"Even then you have to make certain that the docks are safe so that you can offload those materials. Before you can land a C130 or a Chinook you have to make certain that the runway is safe or the landing zone is safe," he said.