The sole survivor of a fiery plane crash in Middletown on Thursday night remained in critical condition yesterday at Rhode Island Hospital.

The identities of the two who died in the crash of the small Piper aircraft have still not been released by officials.

Keith Ulich, 28, of Newport, was pulled from the plane shortly after it crashed at Newport State Airport about 7:30 p.m. and burst into flames. Ulich was transported to the hospital. The Rhode Island State Police said they were in the process of notifying the victims’ next of kin and that the state medical examiner’s office would have to use dental records to identify them, before releasing their names.

Because it was a private airplane, used at times for flight instruction, no flight plan was required or filed, said James Warcup, chief aeronautics inspector with the Rhode Island Airport Corporation.

The single-engine plane is registered to Charles Hallal of Westport, Mass. Hallal was not on the plane, according to Robert Gretz, a senior air safety investigator with the National Transportation Safety Board, the agency that is overseeing the crash investigation. Officials from the Federal Aviation Administration are also participating in the investigation.

Apparently the plane was sometimes leased for flight instruction lessons, Warcup said, although it is unclear whether the trip Thursday night was an instructional or a pleasure flight. The plane appeared to lose power immediately after takeoff, he said.

Gretz said at a news conference Friday that he expects the investigation to take at least six months.