The Maritime and Coastguard Agency launched an investigation into the first spill, but has since said it had been unable to trace the source of the pollution and confirmed it had closed the investigation.
After the first incident, the MCA launched an investigation into the spill, but has since said it had been unable to trace the source of the pollution.
"The FBI is now conducting further forensic examination for the purpose of identifying trace evidence, residues and signatures of production that could provide evidence to support the investigation, " the agency said.
"The FBI is now conducting further forensic examination for the purpose of identifying trace evidence, residues and signatures of production that could provide evidence to support the investigation, " she said in a news release.