Has the NSA been tracking the cellphone movements of millions of Americans?

Senator Wyden suggests that the NSA is either tracking or has been studying tracking millions of Americans.  Presumably Wyden isn't allowed to explicitly say what the NSA is doing, so his suggestions should probably be given more weight than suggestions by others.  From The Hill newspaper:

Sen. Ron Wyden (D-Ore.) suggested on Thursday that the National Security Agency tracked or considered tracking the cellphone location data of millions of people in the United States.  

During a Senate Intelligence Committee hearing, Wyden asked NSA Director Gen. Keith Alexander whether "the NSA has ever collected — or made any plans to collect — Americans' cell site information in bulk." 

As a member of the Intelligence Committee, Wyden has access to classified information about the NSA's surveillance programs. . . .  

Analyzing which cell towers a phone has connected to can allow law enforcement or intelligence officials to track the location of the phone's owner.  

Wyden has been a long-time critic of NSA surveillance, but government secrecy has limited his ability to publicly discuss his concerns. . . .