Here are the top 10 developments on this big story:
Indian High Commissioner in Colombo Taranjith Singh Sandhu has taken up the killing with Sri Lankan Prime Minister Ranil Wickremesinghe and expressed New Delhi's "deep concern" at the killing of the fishermen.
Sri Lanka has told New Delhi that it was keen not to let any incident to stand in the way of good relations between the two countries and vowed a full probe into the incident in cooperation with Indian authorities.
It followed up the promise with a formal statement, that not just shared New Delhi's "deep concern" but also called the safety of fishermen of "utmost importance". It added its agencies "treat all Indian fishermen in a humane manner at all times".
A young fisherman, Bridgo, was fishing near the Kachchatheevu islet when Lankan naval personnel opened fire, according to Tamil Nadu fisheries department. Bridgo died immediately, he was shot in the neck while at least one other fisherman was injured.
The Lankan Navy's denial cited initial reports from Tamil Nadu that suggested the Navy personnel had come in a small boat. The Navy said it did not deploy smaller boats to patrol the northern waters but the larger, Fast Attack Craft and Fast Gun boats, since Indian trawlers that cross over too are "quite large".
Also, it said, Navy personnel were not authorised to open fire without prior approval of the Navy's Commander and only arrested fishermen poaching inside Sri Lankan territorial waters.
Monday's firing took place in Palk Bay, a narrow strip of water that separates Tamil Nadu from Sri Lanka that has been rich fishing grounds for both countries. Tamil Nadu has been nudging New Delhi to do more to put an end to harassment of Indian fishermen who may cross over into territorial waters and retrieval of the tiny island of Kachchatheevu, a land mass no more than 285.2 acres, is strategically important for fishing activities.
In Tamil Nadu, that has often influenced the Centre's hand vis-a-vis Sri Lankan, Chief Minister Palaniswamy today wrote to PM Narendra Modi asking that the Centre use all means to curb unacceptable aggressive actions of Sri Lankan Navy in the Palk Bay and convey the government's strong feelings about the unprovoked firing.
Tamil Nadu says 85 fishermen and 128 fishing boats are currently in Lankan custody and has urged the Centre to take action through diplomatic channels.
DMK's MK Stalin wanted New Delhi to scale up its response, saying "India should warn Sri Lanka diplomatically that this can't be tolerated and there will be strong repercussions".
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