Manmohan Singh in Jamshedpur on Tuesday. (PTI)

New Delhi, April 23: A ghostly blip on the radar that indicated an intrusion into the flight path of the Prime Minister’s aircraft as it was on its landing run for Delhi yesterday evening is still being investigated but two different accounts have deepened the mystery.

In the worst case scenario, the blip could have indicated an object chasing or targeting the aircraft.

Civil aviation minister Praful Patel, however, said in Ahmedabad that “it was not a serious security lapse”. He said an Indian Air Force Dornier aircraft “was at a reasonable distance from the Prime Minister’s plane”.

A source in the air force — that is not giving its official account publicly — said the ghostly blip was the result of atmospheric changes over Delhi as the summer peaks. Such a phenomenon called “total internal reflection” is not unknown. The source said the blip was “spurious”.

The air force is preparing a report that will be given to the investigating authority appointed by the Directorate General of Civil Aviation/Airports Authority of India (AAI).

The IAF flies the Prime Minister from the Headquarters Communication (VVIP) Squadron. The Prime Minister was flying in the IAF’s Boeing 737 from Ranchi last evening. The control of air traffic over the national capital vests with the civilian authorities that man the air traffic control (ATC) with air force controllers embedded in it.

Delhi airport sources familiar with the incident said the blip on the radar appeared to be a “ghost image” which air traffic controllers have spotted in the past.

“It’s happened earlier, and it’s been reported to authorities, but now that it’s happened with the Prime Minister’s aircraft, things are coming to light,” a source said.

The ghost image could have resulted from some other aircraft in the neighbourhood, the source said.

But Patel said: “A primary investigation has revealed that there was some mix-up in the signals. To be sure we are waiting for the detailed investigation report.”

The Prime Minister’s aircraft was delayed by 12 minutes, according to the IAF source. A source in the ATC said it was delayed by 16 minutes. Security agencies in New Delhi’s Indira Gandhi International Airport were put on high alert.

The Dornier aircraft that Patel mentioned was “above and behind” the Prime Minister’s aircraft, an AAI source said. “We cannot say that it was in its flight path,” he said.

The Dornier — an aircraft that flies much slower than a Boeing 737 — are used by the IAF and the Indian Navy and also by private companies.