Two people have been questioned over potential manslaughter offences after a fatal explosion at an oil refinery in Pembroke, Dyfed-Powys Police say.
Four people were killed last June in a blast at the site, which at the time was owned by oil company Chevron.
Julie Schmitz, 54, a fire guard from Pembroke, died alongside Dennis Riley, 52, Robert Broome, 48, and Andrew Jenkins, 33, all from Milford Haven.
Police say questioning is part of the ongoing investigation into the blast.
"Dyfed-Powys Police can confirm that on Tuesday, the 7th of February 2012, they interviewed a second employee of Valero Energy Limited under caution at Haverfordwest police station in connection with the explosion at the refinery in Pembroke," said a spokesperson.
"This person was interviewed in relation to potential offences of manslaughter by gross negligence."
Earlier on Wednesday the force confirmed that a male employee of Valero Energy Limited had been interviewed on a similar basis on January 24.
The deaths happened when a 730 cubic metre storage tank exploded, damaging a vessel next to it.
A fifth person was also seriously injured in the blast.
At the time of the incident, the refinery was operated by Chevron, although a deal had already been struck to sell the plant to Valero.
"These developments are very much part of the continuing joint police and Health & Safety Executive investigation which is of an extremely complex nature, and no conclusions should be drawn from it at this stage," added the police spokesperson.
An inquest into the four deaths was opened and adjourned over the summer, while the full inquiry into the explosion continues.