A U.S. Senator is questioning why the Federal Communications Commission (FCC) approved of a controversial cell phone surveillance device that both federal and state law enforcement agencies are using to track suspects, often without court orders to do so.

The device, called a StingRay, is made by the Florida-based Harris Corp. and is used by a dozen federal agencies, including the FBI, NSA, DEA, the Immigration and Customer Enforcement agency and all branches of the U.S. military, according to the ACLU.

The ACLU also identified 48 other agencies in 20 states and the District of Columbia that own StingRays, but many agencies continue to shroud their purchase and use of of the devices in secrecy; civil rights groups said their numbers likely "dramatically" underrepresent the actual use of StingRays nationwide.

Sen. Bill Nelson (D-Fla.) said in a speech on the Senate floor Wednesday that the proliferation of software and spy devices "poses a grave threat to consumers' cellphone and Internet privacy," and will force lawmakers to grapple with finding new ways to curb privacy intrusions.

In a letter to FCC Chairman Tom Wheeler, Nelson asked what oversight is in place to make sure that use of the StingRay complies with the manufacturer's representations to the agency at the time of certification.

Nelson also called on the FCC to explain its rationale behind the restrictions placed on the certification of the StingRay, and whether similar restrictions have been put in place for other devices.

"When this device is turned over to local law enforcement, are they being adequately trained on...judicial protections?" Nelson said, citing published reports that citizens' constitutional privacy rights are being ignored by law enforcement.

"This is not the last time we're going to be asking these questions. There [is] a multiplicity of devices coming onto the market. The question is, what about our privacy?" Nelson said.

Also known as "cell site simulators" or "IMSI catchers," the Stingray is an invasive surveillance devices that mimics cell phone towers by sending out signals that trick mobile devices in the area into transmitting their locations and identifying information.

In his remarks, Nelson cited a Feb. 24 article in The Washington Post about how the Tallahassee Police Department and other law enforcement agencies have been using the StingRay to collect cell phone call information.

The ACLU has said that when law enforcement and other government agencies use StingRays to track a suspect's cell phone, "they also gather information about the phones of countless bystanders who happen to be nearby."

"The ACLU has uncovered evidence that federal and local law enforcement agencies are actively trying to conceal their use from public scrutiny, and we are continuing to push for transparency and reform," the civil rights group wrote on its website.