Washington (CNN) -- A bipartisan group of senators is asking federal authorities to determine whether mobile phone "stalking apps" are legal.
In a letter to Attorney General Eric Holder and Federal Trade Commission Chairman Jon Leibowitz, the group -- led by senators Al Franken, D-Minnesota, and Charles Grassley, R-Iowa -- pressed for an investigation of smart phone applications that allow people to secretly track others' movements. Some apps also allow users to track the e-mails and monitor the phone calls of their targets.
"Stalking apps are dangerous. We ask that you quickly determine if they are also illegal," the senators wrote.
"If so, we ask that the Department of Justice and the Federal Trade Commission use their full force to investigate and prosecute those behind the development and marketing of these products for illegal stalking," they said.
Nine senators ranging from very liberal to staunchly conservative signed the letter.
Stalking apps are marketed in part to individuals wishing to track their spouses and intimate partners. The senators cite one ad proclaiming: "Worried about your spouse cheating? Track every text, every call and every move they make."
The letter notes that a Minnesota woman was tracked by her abuser as she went between government buildings to obtain a legal order requiring the stalker to stay away from her.
The Bureau of Justice Statistics estimates about 26,000 Americans were victims of GPS stalking last year, and the number is believed to be growing.
Joining Franken and Grassley in signing the letter were Democrats Amy Klobuchar, Richard Blumenthal, Sheldon Whitehouse, Charles Schumer and Dianne Feinstein, and Republicans Lindsey Graham and John Cornyn.
The Justice Department had no immediate response.