Mine officials in Utah have suspended "indefinitely" underground efforts to reach six trapped miners, after a cave-in left three rescuers dead.
A sudden cave-in left three rescue workers dead and six injured
Attempts to locate the men from above ground by drilling into the mountain will continue, but hopes are fading.
The cave-in which killed three mine workers and injured six more resulted from seismic activity, officials said.
No contact has been made with the six trapped miners since a tunnel collapsed 1,500ft (457m) underground on 6 August.
Since the first mine collapse, rescue teams had tunnelled about 250m towards the trapped miners, with about 350m to go, when the latest accident happened at 1835 (0035 GMT) on Thursday.
The collapse was caused by a "mountain bump" - a build-up of pressure inside a mine from overhead rock that forces surrounding rock and coal to shoot out of the walls with great force, experts said.
Richard Stickler, head of the federal Mine Safety and Health Administration (MSHA), announced the indefinite halt to underground rescue efforts at a news conference on Friday.
"Is there any possible way we can continue this underground operation and provide safety for the rescue workers? At this point we don't have an answer," he said.
Mine safety experts are being brought to Utah to study the options, he said.
The best mine safety equipment available had been in use when the "mountain bump" caused the tunnel wall to blast in with the force of an explosion, he added.
The rescue effort will now focus on the fourth bore hole, being aimed at a spot where a listening device lowered down a previous hole heard "noise".
If the miners were found alive, Mr Stickler said, that bore hole could be used to supply them with food and air until a large enough opening could be drilled to lower a capsule to bring the men back up.
He praised those killed and injured as "heroes" who had been willing to make the ultimate sacrifice by risking their own lives in the attempt to save others.
Three rescue workers are still being treated in hospital for their injuries, among them an MSHA employee. One of those killed also worked for the federal body.
Speaking at the same news conference, Utah Governor Jon Huntsman offered his prayers to the families of those killed and injured on Thursday, as well to the relatives of the missing miners.
Relatives of the rescuers waited anxiously outside the mine for news
"Suffice it to say that yesterday we went from a tragedy to a catastrophe," he said.
"We want to make sure that the lives that were lost last night were not lost in vain, that as a result of what we learn from this week-and-a-half of pain we become better and smarter and safer."
Rob Moore, vice-president of Murray Energy Corporation and joint owner of Crandall Canyon mine, said efforts to find and rescue the miners would continue.
"Without question, we have suffered a setback and we have incurred an incredible loss, but this team remains focused on the task at hand," he said.
Huntington Mayor Hilary Gordon earlier told TV news network CNN on Friday: "It just feels like a really hard blow to swallow after all we've been through the last week-and-a-half and everyone trying to cope in their individual way."
Initially the original collapse was blamed on an earthquake, but experts have since suggested the shafts might have caved in with enough force to register on seismographs.
This has led to speculation that mining procedures could have triggered the incident.
The workers may have been using a technique known as "retreat mining", where the last standing pillars of coal are pulled down and the roof is allowed to fall in.
The mine's owners have repeatedly insisted their operation was safe and that the mine collapse followed a natural earthquake.
Do you live in Utah or work in the mining industry? What is your reaction to this story? Send us your comments by filling out the form below.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.