Search teams are racing to find the sub before oxygen runs out. Here’s what you need to know

(CNN) -- The search for the missing Titan submersible is now in a critical stage, as rescue teams race to locate the vessel before oxygen supplies run out.

The submersible begins each trip with 96 hours of life support and has been missing since Sunday, setting up Thursday morning as a key target for finding the vessel and those on board. Officials fear the craft’s oxygen supply could run out this morning.

Medical personnel and search vessels with extra capabilities headed to the scene on Thursday, with time running out.

If you're just reading in now, here's the latest news this morning:

• "New capabilities" in search: New, high-tech vessels and medical personnel are moving to the search site as rescue efforts reach a pivotal moment, a Coast Guard official has said. That includes a Magellan ROV, a uniquely equipped vessel whose use was pushed for by the Explorers Club group early on in the search.
• Search vehicle reaches sea floor: A remote operated vehicle “has reached the sea floor” and has begun searching for the missing Titanic submersible early Thursday morning, according to the US Coast Guard. It added that “The French vessel L'Atalante is preparing their ROV to enter the water.”
• Concern over low-tech features: A former OceanGate subcontractor who worked on the development of the Titan submersible said while the game controller to operate the vessel may seem low-tech, it was actually by design. OceanGate tried to use as many “off-the-shelf" items as possible to cut down on research and development as well as costs, Doug Virnig told CNN Wednesday.
• How the sub went missing: The vessel, operated by OceanGate Expeditions, began its two-hour descent to the wreck of the Titanic on Sunday morning. (See how deep the wreckage is here.) It lost contact with the Polar Prince, the support ship that transported the craft to the location in the North Atlantic, 1 hour and 45 minutes into its descent, officials said. Search operations began later that day. It’s still not clear what happened to the submersible, why it lost contact, and how close it was to the Titanic when it went missing.
• What we know about the noises: Banging noises were identified by Canadian aircraft on Tuesday and Wednesday morning. Remotely operated vehicle (ROV) equipment was relocated to where the noises were detected, according to Capt. Jamie Frederick, the response coordinator for the First Coast Guard District. But searches in the area "yielded negative results," he said. Data from the plane that identified the noises was sent to the US Navy, but has so far been inconclusive," Frederick said, adding that the Coast Guard does not know what the sounds were.
• What it could be like onboard: Officials believe the five people on board have "limited rations" of food and water. Ret. Navy Capt. David Marquet, a former submarine captain, told CNN the near-freezing water at that depth is probably making the situation very uncomfortable. "There's frost on the inside of the parts of the submarine. They're all huddled together trying to conserve their body heat. They're running low on oxygen and they're exhaling carbon dioxide," he said.