MILAN (Reuters) - Investigators have found that two Italian sub-contractors used cheaper and non-compliant forms of titanium and aluminium to make certain parts for Boeing's 787 Dreamliner, saving significant sums of money and threatening plane safety, prosecutors said on Saturday.

Below are details of the investigation:

HOW IT STARTED

The investigation by prosecutors in southern Italy started in late 2021 after Boeing said some titanium parts for its plane, supplied by sub-contractors for state-controlled aerospace and defence group Leonardo, had been improperly manufactured over the previous three years.

PROBE OUTCOME

Investigators found that two Italian sub-contractors used pure titanium instead of the prescribed titanium alloy, as well as aluminium alloys different from those prescribed to make even structural components of the aircraft.

BOEING LAUNCHED EXTRAORDINARY MAINTENANCE CAMPAIGN

The probe concluded that some non-compliant structural components could, in the long run, undermine the safety of the aircraft, prompting Boeing to launch an extraordinary maintenance campaign on the aircraft involved.

The U.S. Department of Justice and the Federal Bureau of Investigation have helped Italy's finance police and prosecutors. Leonardo and Boeing cooperated actively to identify the non-compliant components.

WHO DID WHAT?

Italian prosecutors accused seven people and two sub-contractors of crimes including fraud and breaching airplane safety rules, without naming the sub-contractors or the seven people.

Two sources told Reuters that at the centre of the probe are family-owned Manufacturing Process Specification and its now-bankrupt predecessor company Processi Speciali.

MPS owner Antonio Ingrosso and his father Vincenzo, who headed Processi Speciali, are two of the seven people involved in the probe, the sources said. The two men are "convinced that they have acted respecting fully the law," their lawyer told Reuters.

NON-COMPLIANT AIRPLANE PARTS

Processi Speciali and then MPS supplied parts for sections 44 and 46 of the Boeing 787 Dreamliner's fuselage, which are built by Leonardo in southern Italy.

Court documents had said MPS and its predecessor used to supply Leonardo with fittings that helped secure the floor beam in one fuselage section, as well as other fittings, spacers, brackets and clips for a cargo door frame.

Both Leonardo and Boeing dropped MPS from their suppliers' lists after the probe started.

WHAT'S NEXT?

The people and companies accused by the prosecutors will be given time to present any new evidence in their defence, before the prosecutors decide whether to request a judge to call a trial.