Even this is not enough. The executive has to weigh the intelligence with all other factors and give the go ahead or empower the folks.
I suspect that the last step was found wanting due to cognitive dissonance in the political leadership.
Ok won't/don't believe India warnings.
What about the SL police raid recovered intelligence and the NTJ track record?
Why were these not considered?
Actually the khan Amby's statement "we did not provide any intel" starts to make a little more sense now.
What if (if not the Amby herself, then some of her staffers in "off-the-record" meetings) actually were giving GoSL indications in the other direction?
For example if GoSL security officials were meeting the attache-of-whatever for cocktails... mentioned the Indian intel and asked for corroboration... but were told instead, "oh, pay no attention, that's just Hindootva fundamentalist Modi trying to turn you against peace-lovin' Mzzlmz and Packeestan?"
Let's remember something. Much of the old State Dept is still intact and thinking exactly the way it used to (not) think. The rank-and-file babooze regard Orange-utan appointees like Tillerson or Pompeo with hostility as rank outsiders who need to be defied or undermined. Their default assumptions are exactly what Kissinger, Brezinski, Halfbright, Clinton, Kerry and others have left them stewing in for a generation.
Orange-utan for his own part has been lazy about revamping the personnel in the foreign policy establishment. He has a terribly limited attention span, arbitrary decision making style, and tendency to micromanage a few things while paying zero attention to many other things. He's appointed a few Ambys but many others remain in place from previous administrations, or else simply rise through institutional seniority to fill vacancies, especially ones to smaller "s---hole" countries like SL.
So the rank-and-file of khan foreign policy establishment remains exactly of the same mindset that "lost" Indian-supplied evidence in the 1993 bombing investigation, ignored Indian intel inputs as "motivated by hatred of Packeestan" when warned that IC814 was a dry-run for 9/11, regularly insisted that our intelligence agencies were both unprofessional and tainted by anti-Mzzlm bias, etc.
Added later: khan Amby to SL apparently was appointed in 2018, but previously was amby to Nepal. Maybe she just arrived in Colombo as a matter of institutional rotation rather than being handpicked. OTOH, if she was actually selected for the job by Orange-utan team, it's possible she herself was undermined and is trying to get things under control (claiming "no intel was supplied" is better than admitting wrong intel was supplied).