An underground electric cable fire saw potentially lethal levels of poisonous gas leak into homes in Derry.
Around 25 people were evacuated from terraced houses and a bar in Upper Bennett Street during the night as firefighters worked to establish the source of the carbon monoxide.
They originally treated the incident as a gas leak but later traced the fumes to a burning electric cable under the pavement.
Fire Service District Commander Barry McDowell said officers used thermal imaging equipment to find the fire — at which point engineers from NI Electricity drilled down and removed the cable.
“This was a pretty major incident and an unusual one,” said Mr McDowell.
A 50-metre exclusion zone was put in place with residents out of their home throughout most of the night. Fire crews were on site from 1am to 7.30am.
Mr McDowell said the gas reading recorded in terraced homes during the incident was a lethal one.
“It was a very high level of carbon monoxide, a potentially fatal level,” he said.