Fire marshals determined the blaze "was incendiary - an ignitable liquid was used as an accelerant," officials with the New York City Fire Department tweeted Thursday.
Sources involved in the investigation told ABC News that the blaze, which also left two critically hurt, appears to have been set by a tenant who was facing eviction.
A 76-year-old man was rushed to New York Presbyterian Hospital-Queens where he died, said police.
A 10-month-old boy and 35-year-old woman survived and were taken to Cornell Hospital in critical condition, said police.
"Unfortunately it's a sad afternoon here in East Elmhurst," New York City Fire Department Commissioner Daniel Nigro said Wednesday. "There are two people that remain in the hospital in extremely critical condition with burn and smoke injuries from this fire."
"It's quite unusual at that time in the afternoon to have a fire trap five occupants in a private dwelling," Nigro said. "The first call we received came from the occupant of the first floor apartment in that home who heard an alarm going off and called 911."