Small, sub-surface cracks have been found in three more Southwest Airlines planes like those thought to have caused another to develop a hole in its cabin roof mid-flight, officials say.
Tests on the 57 remaining jets are expected to be completed by Tuesday evening. Further flight cancellations are likely until all are back in the air.
The NTSB said Boeing would be drafting a "service bulletin" to describe the inspection techniques that they would recommend carrying out on similar planes with comparable flight cycles (take-offs and landings) as the one involved in the accident over Arizona, which was 15 years old.
There are 931 such models in service worldwide, 288 based in the US.
"What we saw with Flight 812 was a new and unknown issue," said Mike Van de Ven, Southwest's executive vice-president and chief operating officer.