(Reuters) - Magellan Midstream Partners LP said on Tuesday it was investigating the cause of a nearly 11,000-barrel gasoline spill from two above-ground fuel storage tanks at its Houston-area terminal in Texas during Hurricane Harvey. The leak at the Galena Park terminal is the biggest spill reported so far relating to the storm, which unleashed record flooding in the state in late August, destroying homes and killing scores of people.  The Environmental Protection Agency has said federal and state authorities responded to spills linked to Harvey at about a dozen industrial facilities.  The exact cause of the tank failures is now under investigation,  Magellan spokesman Bruce Heine said. He said the company believed it was related to the flooding. Magellan has cleaned up much of the spill, and recovered an undisclosed amount that escaped off the terminal s property into a nearby ditch and the Houston Ship Channel, Heine said.   Clean-up activities at the facility are continuing and we are currently removing and replacing affected soil,  he said.  The spill occurred on Aug. 31, Heine said.  Magellan had initially reported a smaller volume of gasoline spilled to state authorities, but adjusted its estimate upward after it was able to make a full assessment, he said.  It s Magellan s long-term practice to conservatively report a product release to appropriate agencies and local authorities as soon as we become aware of a potential incident,  he said, explaining the smaller initial estimate.  In other words, we do not wait until absolute confirmation, as we want to give the earliest possible notice.  Magellan has said that much of the rest of its infrastructure has returned to normal after the storm.  