The investigation into problems with Boeing's 787 "Dreamliner" has shifted to Arizona.

The Arizona Republic reported that federal investigators visited Pratt & Whitney Engine Services in Phoenix and Securaplane Technologies Inc. of Tucson on Tuesday.

Both companies are involved in making components of the plane's auxiliary power unit, or APU. The Japanese-made battery believed to be the source of a 787 fire in Boston this month was connected to the plane's APU.

Securaplane makes the charger for the Dreamliner's APU batteries, and Pratt & Whitney produces the APU, the Republic reported.

The 50 Boeing 787 Dreamliners being flown by airlines have been grounded until the NTSB investigation is concluded.