"We haven't ruled out anything at this point," Louisiana Department of Wildlife & Fisheries' Jason Adriance said.
LDWF's Randy Pausina said contacts with menhaden boat operators revealed that no boats were operating in the area during that time.
Click here to see a video of the fish kill.
Adriance said his agency received the first report of the kill on May 13, and sent out a crew to investigate.
"At that point, only a few scattered redfish were seen," Adriance said.
The specimens were so decomposed that they could not be used for testing, he explained.
The following day, a crew found about 2,000 dead redfish and roughly 900 drum floating. Again, no suitable tissue samples could be taken, Adriance said.
Samples from those two days did not show any water-quality issues, he said.
An observation flight over the Breton Sound on May 15 revealed another 3,000 or so dead reds, as well as a freshwater plume that extended from the Mississippi River into the sound.
A fish that had not begun decomposing was found by an on-the-water crew, and samples were taken. Adriance said his agency is still awaiting results.
The following day, only about 30 dead reds were found — as was another plume of freshwater from the Mississippi River, Adriance explained.
A May 17 overflight revealed only a few dead fish and a freshwater plume that had receded, he said.
Rick A. Schillachi, public affairs coordinator with menhaden processor, said the kill was not caused by any menhaden boats.
"Our boats were off to the west of the Mississippi, as were our competitors," Schillachi said.
He said a similar kill was reported along the Mississippi/Alabama border at about the same time. Again, he said no menhaden boats were in the area.