The midair disintegration of a jet engine over suburban Denver Saturday is the latest in a string of failures that has raised alarm among regulators about debris evading shielding that’s supposed to keep broken parts from hitting aircraft.

The incident aboard United Airlines flight 328, which showered neighborhoods with metal debris, appears to have been the fifth in five years in which a fan blade broke and destroyed the front section of the engine, according to accident reports and safety experts. That portion of the engine isn’t as protected as the core areas around the jet turbines that are built to contain material in a failure.

"It’s getting more attention with each fan-blade-out event, resulting in these dramatic pictures showing the core of an engine hanging from a wing,” said Jeffrey Guzzetti, the former head of accident investigations for the U.S. Federal Aviation Administration.