At least 43 people were injured, including two people in critical condition, after a fire at an apartment building in Turtle Bay Saturday morning, according to the FDNY.

Residents of the apartment building, which is located at 429 East 52nd Street, were told by the FDNY to shelter in place after the fire broke out on the 20th floor at around 10:30 a.m.

Multiple people were rescued from the building, fire officials said. Videos posted to social media showed firefighters using a rope in a dramatic rescue.

Saturday evening, Deputy Chief of Training Frank Leeb said their first units were confronted with heavy fire at the front door, which blocked the residents' only passage out of the apartment.

He said it required a "Herculean team effort." Four firefighters attached themselves to a lifesaving rope, a procedure Leeb says is practiced at least once a week.

"FDNY procedures fundamentally value team effort. Individuals execute that and that's what we saw today," Leeb said.

He said the team started by lowering a firefighter on the rope to a victim who was hanging outside of a window and was caught on a child's gate.

The firefighter was lowered to the ground so a second firefighter could attach to the rope.

According to Leeb, that second firefighter was able to hold onto the victim as a third member was lowered with a Halligan bar to get the window gate free.

The team above then lowered the firefighters and victim to the ground individually.

By the time the rescue concluded, a group of firefighters had been able to enter the apartment and control the fire.

The inside team was able to get to a third victim and lowered them to the ground with a firefighter.

"It is an incredibly risky operation when we put our members on rope from the 20th floor, but our training and our preparation is what it's all about," Leeb said.

According to Leeb, there have been over 50 documented successful FDNY lifesaving rope rescues since 1980.

"I cannot emphasize enough the extraordinary work of our members [Saturday] in unbelievably dangerous conditions," FDNY Commissioner Laura Kavanagh said Saturday afternoon.

The fire was caused by a lithium-ion battery, according to fire officials. The FDNY said nearly 200 fires they have fought this year to date were caused by lithium-ion batteries, and six people have died in fires caused by the batteries.

"These fires, they come without warning. When they do go on fire, they're so intense that any combustibles in the area will catch fire. So we've seen secondary fires," FDNY Chief Fire Marshal Dan Flynn said. "This isn't really what we've seen traditionally, where fires are slow to develop. We're encountering a fully developed fire when fire units are arriving."

The FDNY said 11 firefighters were among the 43 people injured as of Sunday morning. The number of people injured is likely to increase as time goes on, the FDNY said.