Relatives cry as they remove the body of Suwan Sudmun, a Thai victim of Monday's bomb blast, at the Institute of Forensic Medicine in Bangkok, Thailand, August 18, 2015. [Photo/Agencies]

Thailand has seen many violent attacks in recent years, particularly through a more-than-decade-long insurgency by Muslim separatists that has left more than 5,000 dead in the country's deep south. Those attacks have never extended to the capital, however.

Bangkok has seen politically charged violence over the past decade; the deadliest, in 2010, killed more than 90 over two months and was centered on the same intersection where Monday's bomb went off. But none of those attacks included a bomb blast that seemed intended to produce mass casualties.

Matthew Wheeler, Southeast Asia security analyst for the International Crisis Group, said the bombing was a "new type of attack for Bangkok" that doesn't bear the trademarks of typical violence over the past decade from political instability or Muslim separatists.

"It is certainly not like politically motivated attacks we've seen in the past which have generally been designed to grab attention but not cause casualties," Wheeler said, adding that he expected it would have "major ramifications for security in Thailand."

Early Tuesday morning, investigators surveyed the damage as police and soldiers guarded the area, still littered with shattered glass and other debris. The normally busy intersection that was closed off to traffic and eerily empty aside from onlookers standing behind police tape to take pictures. Barricades were set up outside five-star hotels in the neighborhood and security stopped cars to inspect trunks before letting them pass. The intersection was reopened by midday.

At least 20 people were confirmed dead and 140 injured, according to the Narinthorn emergency medical rescue center.

National chief of police Somyot Poompanmoung said the bomb was made with a pipe wrapped in cloth and weighed 3 kilograms (more than 6 pounds).