Francis Justin, who had alerted the coastal police after Italian marines had shot two fishermen two weeks back, and a fellow fisherman became victims of another tragedy that hit Kollam on Thursday.
Justin and Xavier, residents of Kollam district, were killed after an unidentified oil tanker rammed their boat, Don, 13 nautical miles (24 km) off the Alappuzha coast around 3am.
Three fellow fishermen who had sailed with them were missing at the time of going to press, but two others were rescued by other boats in the vicinity.
Navy and Coast Guard ships have been pressed into rescue operations and also to identify and capture the oil tanker.
"The ship was cruising at a high speed. We tried to alert it several times, but it rammed us badly and sped away," said one of the two survivors, Joseph.
The boat had left the Neendakara harbour on Sunday.
Kollam collector PG Thomas said, "Navy and Coast Guard ships, and helicopters are scanning the area to trace the missing ship. Efforts are on to locate the missing fishermen."
The incident triggered panic in the fishing community, which demanded security in the seas. Chief minister Oommen Chandy called a meeting next week to discuss the issue.
"We have to avoid such incidents," he said.
Union minister for shipping GK Vasan announced a probe into the incident.
Alappuzha police have registered a case against the unidentified ship.