Two employees have been questioned in relation to possible manslaughter charges following the horrific explosion at Pembroke Refinery last year.

The first was interviewed last month, while Dyfed Powys Police confirmed this week that on Tuesday, they had interviewed a second employee of Valero Energy Limited, under caution at Haverfordwest Police Station, in connection with the explosion which occurred on June 2, 2011.

Four workers were killed and one seriously injured in the blast at the refinery, which at the time was operated by Chevron Limited.

Julie Schmitz, 54, a fire guard from Pembroke, died alongside Dennis Riley, 52, Robert Broome, 48, and Andrew Jenkins, 33, all from Milford Haven.

Referring to this week's interview, a spokesman for Dyfed Powys Police said: "This person was interviewed in relation to potential offences of manslaughter by gross negligence.

"This is the second person interviewed under caution during this enquiry, the first employee of Valero Energy Limited, also interviewed in relation to potential offences of manslaughter by gross negligence on January 24."

Added the spokesman: "These developments are very much part of the continuing joint police and health and safety executive investigation which is of an extremely complex nature, and no conclusions should be drawn from it at this stage.

"This is part of the due process of law and designed to protect the rights of all persons involved in the enquiry."