Kenyan security forces swept into an upscale shopping mall late Sunday to try to end a two-day standoff with heavily armed Islamist militants after a gruesome attack that reflected the surprising resiliency of one of Africa’s most brutal insurgent groups.

Authorities later said that most hostages had been freed, but they provided few details. It appeared that at least some members of Somalia’s al-Shabab militia, which asserted responsibility for the attack, were still holed up early Monday in the Westgate Premier Shopping Mall, where they killed 62 people in the deadliest terrorist attack in Kenya in 15 years.

“Most of the hostages have been released, and the Kenya Defense Forces has taken control of most parts of the building,” a Kenyan military spokesman, Col. Cyrus Oguna, told the television station KTN. He did not say how many hostages had been held or freed.