The Nautical Institute has issued a Mars Report as follows:
On a towboat, a vessel crew member intended to test the emergency fuel oil shut-off cables. He opened the panel door that contained both the emergency fuel oil shut-off and the CO2 release handles. He activated what he thought was the fuel oil shut-off but instead the CO2 was released.
Fortunately, the audible alarm system and release time delay functioned as intended, allowing personnel to safely evacuate the machinery spaces prior to the CO2 discharge.
Source: Nautical Institute