PHILADELPHIA (WPVI) — A fire damaged 40 decommissioned SEPTA buses, including 15 electric buses, at a Philadelphia storage yard on Thursday.

Employees reported the fire around 6:15 a.m. at the SEPTA Midvale District, adjacent to the Roberts Yard, located on the 2400 block of Roberts Avenue in the Tioga-Nicetown area of the city.

It was upgraded to a 3-alarm blaze around 7:30 a.m. Thick, black smoke could be seen billowing in the air at the height of the fire.

When the smoke finally cleared, some of the buses could be seen melted to the ground.

A total of 100 buses were in the yard, comprising both gas-powered and electric models. They were set to be sold for a “nominal amount” or scrapped, according to SEPTA Interim General Manager Scott Sauer.

Decommissioned buses are generally stored in the area for a year or less while SEPTA makes arrangements to dispose of them.

The electric buses that were involved were manufactured by Proterra and had not been in operation for several years. Sauer mentioned that SEPTA is currently engaged in ongoing litigation with Proterra, and the buses were being stored as part of this legal process.

When they were put into storage, Sauer said the batteries were disconnected, the coolant was drained and the buses continued to be inspected in accordance with manufacturer recommendations.

In 2022, a coolant issue was to blame for a fire in another Proterra bus located at a different depot.

However, the cause of Thursday’s fire is not yet known. The fire marshal is investigating.

No injuries were reported.

Asst. Fire Chief Charles Walker said during a news conference that firefighters encountered several challenges, including access to the fire scene and water supply issues.

Walker said SEPTA workers were able to move buses that were not yet on fire so firefighters were able to get to the flames.

Once they were able to get more water to the scene, crews were able to make progress extinguishing the fire.

Explosions were heard during the fire, which Walker attributed to tires bursting from the extreme heat.

The ATF responded because the K-9 training center is on-site with small amounts of explosives. All explosive devices were quickly removed.

Midvale Depot is SEPTA’s largest facility, which is used for both maintenance and storage. Some 600 employees work there on any given day.

The fire sent a large plume of smoke into the sky, and the Philadelphia Department of Public Health said pollutants have been detected in the air.

Preliminary data collected from the scene shows high levels of PM2.5 and PM10, and shows the presence of benzene, ethylbenzene, and CO, according to health officials. The levels of these pollutants have been dropping throughout the morning, and are expected them to continue to drop.

RELATED: PM10 vs PM2.5: What to know about the particulate matter in the air

Officials said Thursday night, the area is still unhealthy to breathe, and continue to recommend that residents stay out of the area and, if you are in the area, to stay indoors with the windows and doors closed.