A bomb exploded at a popular open-air shrine located at a key Bangkok intersection Monday evening, killing at least 20 people and injuring 140 others. Here is a summary of what we know and don't know.

WHAT WE KNOW:

An improvised bomb blew up around 7 p.m. Monday at the Erawan Shrine, an open-air Hindu temple frequented by tourists and local residents, near a busy intersection in the heart of Bangkok. The area is surrounded by shopping malls and hotels.

In addition to the human casualties, the blast shattered nearby shop and office windows and burned motorbikes but there was no visible structural damage to nearby buildings.

Police chief Somyot Poompanmoung says the bomb was made with a pipe wrapped in cloth and weighed 3 kilograms (just over 6 pounds). He said it had a 100-meter (330-feet) blast radius.

The elevated Skytrain, which runs right by the site, was operating and the intersection that had been blocked off was re-opened to traffic shortly after noon Tuesday — 17 hours after the blast.

WHAT WE DON'T KNOW:

No one has claimed responsibility for the attack, nor has a motive emerged.

Prime Minister Prayuth Chan-ocha said authorities are trying to find suspects after viewing security camera footage.

Analysts have identified three possible motives — Thailand's decade-long political upheaval, a Muslim insurgency mostly in southern Thailand and international terrorism — but authorities say it was too soon to link the attack to any of them.