By Alina Selyukh and Marina Lopes WASHINGTON (Reuters) – The top U.S. communications regulator on Friday said he is asking all large U.S. wireless carriers to explain how they decide when to slow download speeds for some customers, after questioning Verizon Wireless about such a plan. Verizon, the biggest U.S. carrier, said last month that the top 5 percent of high-speed data users on its older unlimited data plans might experience slower speeds starting in October. In a letter to Verizon, Federal Communications Commission Chairman Tom Wheeler said he was deeply troubled by the plan and expressed concern the decision to slow data was based on consumers plans instead of network needs. Verizon defended the practice as a narrow, lawful and widely accepted way to manage networks and said all its competitors also relied on throttling of top data users to optimize congested networks.