In a succinct one-page ruling, U.S. District Court Judge Lynn N. Hughes of the Southern District of Texas declared that the law authorizing the government to obtain cellphone records without a search warrant was unconstitutional.

"The records would show the date, time, called number, and location of the telephone when the call was made," Judge Hughes wrote in the decision, dated Nov. 11. "These data are constitutionally protected from this intrusion."

Judge...