A car packed with explosives being driven to carry out an attack collided with other vehicles and exploded in central Cairo outside Egypt’s main cancer hospital, killing at least 20 people, the Interior Ministry said Monday.

Authorities had initially said the deaths were caused by a multi-vehicle accident Sunday night on the busy Corniche boulevard along the Nile River, with no explanation of how it caused an explosion that damaged the hospital’s facade and even rooms inside.

But later Monday, the ministry later acknowledged that a car bomb was involved. It accused a militant group known as Hasm, which has links to the outlawed Muslim Brotherhood, saying it was moving the car — stolen in the Nile Delta — to carry out an attack elsewhere. The ministry did not say what the intended target was.

Read more: AP