NEWORLEANS- While BP tries to cap the leaking well and responders attempt to keep the bulk of the giant oil spill from hitting the coastline, there is also an investigation underway into exactly what caused the explosion on the Deepwater Horizon.

Witnesses say they saw a huge gush of water shoot out of the derrick on the rig, followed by a large explosion and fire.

'If you read some of the eyewitness accounts, the first thing they noticed was this 300-foot geyser through the middle of the rig that was topping the crown block which is the very top of the derrick,' said Eric Smith, Associate Director of Tulane University's Energy Institute. 'That was the gas pushing the saltwater out of that 21-inch diameter unit.'

Smith spent a long career in the offshore petroleum industry before joining Tulane.

From all accounts, the exploration rig was in the process of capping the well when the rig suddenly exploded.

Smith says workers were probably in the final stages of removing the drilling mud and plugging the well, when one of the valves used to shut off the pipe for some unknown reason didn't close all the way, allowing gas to rise to the surface and ignite.

'My scenario would say that something went wrong in the completed well, either the casing blew out, where they had put in a liner or the cement job failed because they cemented over some drilling mud and the mud washed out and there was a route to the surface,' said Smith.

'I'm not sure that anyone knows that they took the mud out, improperly,' said New Orleans Attorney Scott Bickford. 'We know that the mud had a lot of gas in it as it was coming up. We know that they were replacing the seawater as they were going along and the intention was to cap this well and move off and drill another well.'

Bickford filed the first law suit after the Deepwater Horizon blew up April 20.

'I think there's going to be one cause as to why this huge column of gas got in,' said Bickford. 'I think the other issues are going to be that the safety measures that should have been in place, either failed or weren't in place at the time this happened.'

BP, which owns the well, will likely have most of the potential liability for the spill.

Companies like Halliburton, responsible for the cementing operation on the rig and Cameron which provided the blow out preventer equipment that failed are other likely targets.

The Coast Guard and federal Minerals Management Service are expected to begin hearings next week into the cause of the explosion.