Mount St Helens volcano in Washington State is likely to erupt again very soon, scientists have warned.
More energy was released last week than at any time since 1980
The threat level has been raised to "volcano alert" - the highest eruption risk level indicating there is a risk to life and property in the area.
On Saturday, hundreds of visitors were evacuated from the Johnston Ridge Observatory, 8km (five miles) away.
The volcano erupted on Friday for the first time since 1986. In 1980, a much stronger eruption killed 57 people.
"An eruption could happen right now, in a few days or a few weeks," Catherine Puckett, a spokeswoman for the US Geological Survey (USGS) in Seattle, said.
Federal authorities are evacuating everyone from a 8km radius around the mountain.
Another USGS geologist, Tom Pierson, said the volcano had released more energy in the last week than at any time since the tragic 1980 eruption.
But scientists have reassured the public that the imminent blast will be much less powerful.
The closest community to Mount St Helens is 50km (30 miles) away.
On Friday, the volcano spewed a plume of steam and ash thousands of metres into the sky and destroyed the closest seismic station.
A second steam emission on Saturday was followed by a tremor signal which was picked up throughout western Washington and into central Oregon and prompted the heightened alert level.
It lasted about an hour and after this period of tremor, which is a continual signal of rumbles beneath the volcano, the seismicity returned to discrete earthquakes.
A further tremor sequence lasting 25 minutes occurred on Sunday but there was no evidence of an associated steam emission.
These tremor episodes signify a change in the character of the seismic signals and researchers believe new magma is entering the volcano's upper levels, and that volatile gases it carries with it could lead to further eruptions.
The pattern of seismicity has now returned to the level it was at before the ash and steam eruptions - magnitude three earthquakes are occurring at a rate of every five minutes.
Visual observations and photographic analysis show evidence of uplift of the scale of tens of metres of part of the glacier in the crater and of the lava dome.
The Cascades Volcano Observatory (CVO) report an increased likelihood of larger and more ash-rich eruptions in their most recent alert bulletin.
Wind forecasts from the National Oceanic and Atmospheric Administration (Noaa) show that these ash clouds would drift to the west and northwest and if these ash emissions are large they could affect downwind communities.
The CVO also report a further possible threat to the public could arise from the minor melting of the glacier. This could trigger debris flows from the crater. It is unlikely however that the downstream communities would be affected.
The weekend's events attracted scores of tourists in search of a once-in-a-lifetime experience.
"We wouldn't miss something like this - it's so close," said Martin Stoc, from Vancouver, Washington, who was at the observatory with his family before the evacuation.
When Mount St Helens erupted on 18 May 1980, the upper third of the mountain was blown off.
Gray ash buried towns and cities across the Pacific Northwest, and forests and meadows were devastated.