An administration official on Wednesday credited a routine security screening carried out early this year with helping authorities to track down Times Square bombing suspect Faisal Shahzad.

When Shahzad returned to the United States from Pakistan in February, he qualified for secondary, or more detailed, screening under Customs and Border Protection criteria and was interviewed, the administration official told CNN.

After that screening, CBP followed protocol and sent a report to the FBI and other intelligence agencies that included Shahzad's passenger information, the official said. Included in that report were phone numbers associated with his travel, when he bought his ticket, and when he filed a customs form, the official said.

Last weekend, as they investigated the failed bombing attempt, FBI agents turned up a telephone number but no name in connection with Shahzad's purchase of a Nissan Pathfinder. When agents searched databases containing the numbers called by or to that phone, they found the number on the CBP report, the official said. That is how they came up with Faisal Shahzad's name.

The source could not say whether the number that was the "key" was a domestic number or an international one.

- From CNN Homeland Security Correspondent Jeanne Meserve