As experts examine underwater recordings of noises in the North Atlantic Ocean where a submersible with five people aboard went missing Sunday, the search for the vessel grew Wednesday. With oxygen supplies on board likely dwindling, time has become the biggest concern for rescuers.

The vessel was carrying a pilot, three wealthy adventurers and the submersible company’s CEO when it vanished Sunday morning about an hour and 45 minutes into its trek 12,500 feet deep, where the wreckage of the Titanic rests.

The sub’s 96-hour emergency air supply could run out soon, setting up Thursday morning as an important search target. It remains unclear whether the noises were heard Tuesday night and Wednesday morning were from the missing submersible, US Coast Guard Capt. Jamie Frederick said at a midday news conference. The banging on Tuesday first came every 30 minutes and was heard again four hours later, according to an internal government memo update on the search obtained by CNN.