Dick Costolo will step down as CEO of Twitter after almost five years, the company announced Thursday in a news release. It did not disclose a reason for his departure.

As Twitter Inc. announced the move, Costolo also tweeted the news through his own account.

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The San Francisco company says co-founder and chairman Jack Dorsey, 38, will be its interim CEO while it looks for a replacement.

Costolo, 51, will stay on Twitter's board of directors. He has been a director since September 2009 and became CEO in October 2010.