Two men were shot dead and a third critically injured in a suspected revenge attack carried out by 12 to 15 assailants on motorbikes and a Tata Sumo in Madhyamgram, around 8km from the airport, on Thursday evening.

Babu Sen and Nungkai, the two men killed in the attack, are said to be part of a landfill syndicate that feeds on the booming real estate business in the Madhyamgram-Barasat belt and road-widening projects in the area.

The police suspect that the attack was to avenge two murders near Agarpara railway station, on the northern fringe of Calcutta, last August. Babu is an accused in that case.

The duo, along with associate Abhijeet Guha, were in a car headed for Belghoria on BT Road when the attack occurred at 6.30pm. Abhijeet took three bullets but survived the attack along with driver Jagadish Sarkar. He was admitted to RG Kar Medical College and Hospital.

"Abhijeet took a bullet on his chest and two more on his hands," said a doctor at RG Kar hospital.

Investigators retrieved at least 11 empty cartridges from the foot of the flyover that goes across Madhyamgram railway station and hits the single-lane thoroughfare leading to Sodepur, on BT Road. More bullets might be lodged in the bodies of the slain persons, the police said.

"We need to wait for the post-mortem report to know how many bullets the two victims took," an officer added.

Driver Jagadish, who escaped unscathed, said a Tata Sumo and several bikes blocked the path while he was about to steer the Maruti Alto 800 up the flyover in Madhyamgram. Then the hail of bullets started.

"They fired at me too but I didn't get hit. There were 12-15 people on many bikes and one Sumo, as far as I can recall. I don't know what Abhijeet does for a living but the two persons who died were engaged in landfill deals and construction. We were going to Belghoria when the incident happened," the driver said.

Babu and his associates had apparently surveyed a plot of land in Badu, around 4km from Madhyamgram town, earlier in the evening.

"Babu and Nungkai were engaged in the business of collecting landfill material from fisheries and agricultural land in North 24-Parganas and supplying it to residential and commercial project sites in Barasat, Madhyamgram, Sodepur, Agarpara and Titagarh. Besides, they used to decide which contractors would get supply contracts for highway projects in North 24-Parganas," a police officer said.

The Alto 800 in which the duo were travelling with their associate on Thursday evening had just taken a U-turn to get on the flyover from Devigarh when it was waylaid.

Witnesses to the attack couldn't confirm how many people carried out the attack but some said they saw at least four persons opening fire, leaving the two victims riddled with bullets in a matter of seconds.

The road where the attack occurred - it connects Madhyamgram Chowrasta on Jessore Road with Madhyamgram railway station - is perennially busy and is lined by shops and markets on both flanks. Madhyamgram municipality is barely a kilometre from the spot and the police station is 1.5km away.

Some of the witnesses told the police that the assailants collected a few spent cartridges before fleeing towards Panihati.

The police said the findings of their preliminary investigation pointed to a rivalry between two groups fighting for the spoils of the landfill business. The two victims in the Agarpara murder case in which Babu was an accused had also been engaged in the real estate supply business.

"Surajit Chakraborty and Partha Das used to supply sand and stone chips to construction sites in Agarpara and Ghola. They were murdered on August 15 last year. It seems associates of Surajit and Partha carried out the attack on Thursday," said an officer at Madhyamgram police station.

The area between Madhyamgram railway station and Jessore Road has witnessed a boom in real estate over the past few years. This stretch alone has at least 50 apartment projects, including standalone five-storey buildings and a couple of medium-sized complexes, in various stages of construction.

On the other side of the railway station, at least 60 highrise projects are coming up.

"Some of these projects are in lowland and the plots need to be filled up to attract buyers. Both the groups involved in Thursday's incident had been competing for contracts to supply mud and soil to these projects," said an officer probing the shooting.