WASHINGTON • Details about why Iran air defence forces mistook a Ukrainian airliner for a cruise missile remain murky, but one thing is clear: Safeguards for operating surface-to-air missiles are supposed to prevent that kind of mistaken identity, and all of them failed.

The error, which killed all 176 people aboard the plane last Wednesday, is probably the result of multiple layers of failure, said the Teal Group's senior analyst for missile systems Steven Zaloga.