Several abnormal signs have been found at the scene of an apartment fire that left 13 people dead and at least 50 others injured in Ho Chi Minh City at first light on Friday, police said.

The blaze broke out in the basement of the 21-story Carina Plaza apartment building on Vo Van Kiet Street in District 8 at about 1:30 am, with its smoke and heat reaching as high as the 14th floor.

Most residents were sleeping at the time and were unalarmed as neither the fire alarm nor water sprinkler systems of the building, put into use in 2009, were activated.

As of Friday afternoon, 13 people, including three children, were confirmed dead while at least 50 others were hospitalized in the city’s worst fire in the last 15 years.

Eleven of the deceased victims died of smoke inhalation, while the other two fatally fell during their escape from the building.

Luu Le Bich Phuong, chairwoman of Nguyen Thai Binh Ward of District 1, was confirmed as being among the deceased.

Phuong managed to let her son climb down from their apartment on the 19th floor to the 12th floor using a rope ladder.

When it was her turn, she fell to death after reaching the 17th floor, according to her son.

A victim receives treatment after the fire. Photo: Tuoi Tre

Phan Anh Minh, deputy director of the Ho Chi Minh City police department, said officers had performed an autopsy on the deceased and are examining the scene.

The blaze could have been caused by a vehicle catching fire in the basement, but the possibility of an explosive deliberately placed there could not be ruled out, Minh told reporters on Friday morning.

“The most important question here is why a fire that started in the basement could kill people in the upper floors?” Nguyen Thien Nhan, secretary of the municipal Party Committee, questioned at the press meeting.

Multistory buildings are expected to have fire doors on each floor to reduce the spread of fire and smoke between separate compartments.

However, police found that the fire doors on all floors of the 21-story Carina Plaza building were held open with big stones put at their feet, allowing toxic smoke and heat to travel all the way from the basement to the top floor, Minh said.

When power for the building was cut following the fire, the emergency lights on some floors did not automatically switch on as they were supposed to do, the police officer added.

Block C of the Carina Plaza is seen on fire in Ho Chi Minh on March 23, 2018. Photo: Tuoi Tre

The fire broke out at Block A of the three-block apartment complex. Affected residents have been temporarily relocated to Block C and the CityGate apartment building that sits just opposite the Carina Plaza.

The Carina Plaza residents were, however, left in shock again around noon as it was Block C’s turn to catch fire. The latest blaze was quickly put out, with an overheating pumper, put to use to fight the fire at Block A, identified as the cause.

District 8 authorities have given urgent aid worth VND15 million ($660) to each family of the deceased, and up to VND5 million ($220) to those injured.

Affected victims take a rest on the building campus after the fire. Photo: Tuoi Tre

People leave Block C of the Carina Plaza following the fire alarm. Photo: Tuoi Tre