Police officials in South Korea opened an investigation into the cause of the deadly incident Saturday that killed 154 people during Halloween celebrations in Seoul.

They announced a 561-member task force would interview witnesses to the tragedy to find out what exactly caused so many to be crushed in the crowds of revelers. Police are also analyzing video footage from security cameras and video clips on social media sites.

South Korean Prime Minister Han Duck-soo on Monday promised to probe the incident after so many died in an incident considered the country's worst mishap in recent years.

"The government will undertake a thorough investigation into what caused this accident and do its best to make necessary institutional changes so that such an accident is not repeated," Han said.

He added, "We will do our best to provide necessary support by reflecting the opinions of the bereaved families as much as possible."

What do we know about the deadly celebrations so far?

The tragedy Saturday occurred in a sloped, narrow alley in Seoul's Itaewon neighborhood, known for its nightlife.

Witnesses recalled that the area was jam-packed with vehicles and partygoers, mostly in their teens and twenties, clad in Halloween costumes.

According to officials, the death toll jumped overnight to 154 with 149 injured.

Of the 149 injured, 33 are in critical condition.

People who died during the stampede inlcuded citizens from at least a dozen countries.

State of national mourning

President Yoon Suk-yeol paid his respects to the dead at temporary mourning sites and on Sunday declared a one-week national mourning period.

The government said 153 of the 154 dead people have been identified so far.

President Yoon Suk-yeol declared a one-week national mourning period

Just a few steps away from the site of the mishap, people laid white flowers, drinks and candles at a small makeshift altar.Jung Si-hoon, a retiree and a church elder, told Reuters, "It doesn't matter how they died, or why they died. Those poor people, all at similar ages to my grandchildren, they died anyway," said

Bereaved families are expected to begin funerals of their loved ones as the process of identifying victims is near complete.