Three U.S. senators, including Democrat Amy Klobuchar who chairs the Senate Judiciary Committee's antitrust panel, wrote a letter to Amazon.com Inc to express concern about its expansion and promotion of a palm print recognition system, Klobuchar's office said Friday.

Amazon began rolling out biometric technology at its Whole Foods stores around Seattle in April, letting shoppers pay for items with a scan of their palm. The system, called Amazon One, lets customers associate a credit card with their palm print.

Klobuchar, who was joined by Senators Bill Cassidy, a Republican, and Jon Ossoff, a Democrat expressed concern in the letter dated Thursday about both privacy issues and competitive ones related to the program, called Amazon One.

"Our concerns about user privacy are heightened by evidence that Amazon shared voice data with third-party contractors and allegations that Amazon has violated biometric privacy laws," the lawmakers wrote in the letter.

"We are also concerned that Amazon may use data from Amazon One, including data from third-party customers that may purchase and use Amazon One devices, to further cement its competitive power and suppress competition across various markets," they wrote.

The lawmakers asked Amazon to tell them about plans to expand Amazon One, and to whom they had sold or licensed the technology. They also asked how many people had signed up for it, how the data is used and if it is ever paired with facial recognition systems. (Reporting by Diane Bartz; Editing by Nick Zieminski)