Police don't need a warrant to search a cell phone for its number, a federal appeals court has ruled.

The decision (PDF), issued by the U.S. Court of Appeal for the 7th Circuit, stems from an Indiana case in which prosecutors used evidence that police found on cell phones at the arrest scene to convict a suspect on drug charges.

Police had subpoenaed three months of each cell phone's call history to gather evidence on one of the defendants in the case, Abel Flores-Lopez. Defense attorneys appealed their client's 10-year prison sentence, arguing that the police should have requested a search warrant before searching Flores-Lopez's phone. They maintained that any evidence obtained from the phone company thus was the fruit of an illegal search and therefore should be ruled inadmissible.

The three-judge panel was unpersuaded, likening the cell phone to a diary.

"It's not even clear that we need a rule of law specific to cell phones or other computers. If police are entitled to open a pocket diary to copy the owner's address, they should be entitled to turn on a cell phone to learn its number," they wrote in the opinion.

"So opening the diary found on the suspect whom the police have arrested, to verify his name and address and discover whether the diary contains information relevant to the crime for which he has been arrested, clearly is permissible; and what happened in this case was similar but even less intrusive, since a cell phone's phone number can be found without searching the phone's contents, unless the phone is password-protected--and on some cell phones even if it is."

The court offered up a scenario in which they said that the defendant's associates conceivably could have learned about the arrests and remotely wiped the cell phones clean before the government was able to get a warrant permitting investigators to conduct a search.

They described the authorities' ability to obtain the device's phone number quickly as a "modest cost" in privacy invasion.

"Armed with that number the officers could obtain the call history at their leisure, and the defendant does not deny that if the number was lawfully obtained the subpoenaing of the call history from the phone company was also lawful and the history thus obtained could therefore properly be used in evidence against him," the judges wrote.

A similar issue surfaced around a 2004 arrest by San Francisco police as part of a marijuana investigation. The police searched the mobile phones of three of the five men they arrest but without first requesting a warrant. U.S. District Judge Susan Illston later ruled that the SFPD's warrantless search was not permissible.