Story highlights A US Airways Express crew reports seeing what looked like a flare with a smoke trail

The sighting occurred while the aircraft was on landing approach in Philadelphia

Authorities say the sighting remains a mystery

A US Airways Express flight crew reported seeing what looked like a flare with a smoke trail in the vicinity of their aircraft while on approach to Philadelphia International Airport on Tuesday.

According to authorities, what the crew witnessed remains a mystery. The aircraft with 34 passengers and three crew members landed safely.

Flight 4321, originating from Elmira-Corning Regional Airport, was about 500 feet above the ground in Philadelphia when the incident took place.

An audio recording of the communication provided by Live ATC.net between the air traffic controller and arriving planes indicated the initial confusion:

Pilot: "It looks like a flare that was shot out."

Tower: "Previous arrival said something about a flare being shot up. ... You said there was a flare being shot at you?"

Pilot: "Yeah it wasn't probably more than 50 feet off our right wing. It looked like a flare gun."

Tower: "OK, wow."

The tower alerted incoming aircraft to use caution as they approached the airport. No other incidents were reported.

After landing in Philadelphia, the aircraft taxied to the gate, according to US Airways spokeswoman Liz Landau.

Runway 17 was closed for about 30 minutes after the incident for investigation, the FAA said. Law enforcement authorities are investigating.

The aircraft involved was a Bombardier Dash 8 twin engine plane operated for US Airways by Piedmont Airlines.