A pulsing signal has been detected from under the rubble of a building that collapsed in an explosion at the port of Beirut last month, raising hopes that there may be survivors.

A sniffer dog, part of a Chilean search and rescue team, first identified something in the rubble of a building that had been searched earlier. The team then used audio equipment to listen to potential heartbeats and identified what could be a pulse of 18 to 19 beats per minute.

One month after the blast, no survivors were found, killing 191 people, injuring thousands more and damaging thousands of homes. The last few weeks have been extremely hot in Lebanon with high levels of humidity.

“99% is nothing, but with less than 1% hope, we should look for it,” said Civil Defense Officer Yusuf Malah, adding that his men would work all night.

Francesco Lermonda, a Chilean volunteer, said their equipment detected breathing and heartbeat from humans, not animals, and it identified a mark on the site. He said it was rare for anyone to survive under the rubble for a month but it was not heard.

As night fell, rescuers set up lights to work through the darkness. Each time and later, the Chilean team told outside visitors to turn off their phones and keep quiet while listening to life signs.

French experts as well as the FBI have taken part in the investigation into the blast at the request of Lebanese authorities. Their searches have not yet been published. Authorities detained 25 people, most of them port and customs officials, in connection with the blast.