AT&T Inc. (NYSE: T - message board) says it has concluded its investigation into what caused a broadband equipment cabinet to explode in suburban Houston last year. (See AT&T Investigates DSLAM Explosion and AT&T Still Digging on DBLAM!) The verdict: The whole shebang was caused by a manufacturing defect related to the cabinet's battery.

    Some quick background: The VRAD (video ready access device) that went off last October was located in the 8200 block of Clover Gardens Drive, outside the home of an elderly couple. (See AT&T Eyes Batteries in Explosion Probe and Exclusive Photos: Fire to the Node.) The cabinet itself contained a battery for backup power made by Avestor, an AT&T supplier that went bankrupt in 2006 and shut down shortly thereafter.

    There are 17,000 of the same type of Avestor batteries across AT&T's network. But AT&T says a thorough investigation by engineering firm Exponent Inc. has put to rest fears that future incidents are likely.

    Here is AT&T'sstatement in full:

    After one of our network equipment batteries malfunctioned in Houston last year, we retained one of the nation's leading engineering and scientific consulting firms to investigate the issue, along with a subsequent incident, and advise the company as to the risk of continued usage of the battery in question.

    The final report was issued [to AT&T] last week. The third-party firm found that the battery design was sound, as were the safety features, and concluded that the risk of hazardous failures with this battery is as low, if not lower, than the risk with alternative batteries, which are used by other telecommunications and cable companies in similar applications. The investigation found that the incidents in Houston were most likely caused by a manufacturing defect in those two batteries.