The team investigating one of the 2017 London Bridge attackers was not told he had been reported to an anti-terror hotline, an inquest has heard.
Khuram Butt's brother-in-law had reported his increasing radicalisation in September 2015.
In the same month MI5 assessed Butt as wanting to stage a terror attack but lacking the ability to do so.
Eight people were killed in the attack he carried out with two other men.
They mowed down pedestrians on London Bridge before launching a knife attack in nearby Borough Market, injuring 48 others.
Police shot and killed the attackers less than 10 minutes after the violence began.
A senior counter-terrorism officer - identified only as Witness M - told an inquest at the Old Bailey in London that it was "very unsatisfactory" his team was not informed about the call.
Usman Darr had contacted the hotline because he was concerned that his brother-in-law had been distributing anti-Western texts and links to jihadi sites and had become increasingly extreme in his views.
The information was processed but never passed on to the joint MI5 and police investigation of Butt that had been under way since mid-2015.
In the same month that he was reported by his brother-in-law, Butt was assessed by the security service as having a "strong risk" of staging a terror attack on his own, but there was no evidence he was planning one.
In May 2017 MI5 lowered the assessed risk of Butt carrying out a lone terror attack to moderate, but increased his ability to do so to moderate.
Police decided not to charge him with possession of extremist material because there was not a strong enough chance of disrupting any potential terror plot, Witness M said.
The inquest also heard Butt had associated with members of the banned terrorist group Al-Muhajiroun, including Siddhartha Dhar - who went on to fight for so-called Islamic State - and the group's leader Anjem Choudary.
Witness M also told the inquest he had not personally been made aware that Butt had appeared in a Channel 4 programme called The Jihadi Next Door in January 2016, saying the programme was reviewed by another team.
The court previously heard how Butt appeared in the programme - where he condemned the UK government, particularly over its actions in Iraq and Syria - for roughly two minutes but was not identified by name.
Later that year, Butt was employed by London Underground, including working at Westminster station, but Witness M said he did not have grounds to intervene.
In October 2016, Butt was arrested with three others on suspicion of falsely reporting fraudulent activity on three bank accounts, and bailed until January 2017.
However, prosecutors advised there was not enough evidence to charge him.
Witness M said police were also unaware of a number of pieces of information that indicated Butt was associated with the two other attackers.
These included the fact they all met at Ummah Fitness Centre in Ilford, east London, that was itself run by a suspected senior member of Al-Muhajiroun, and that they went on regular trips together to take their children swimming.
Last week, the lawyer representing several of the victims' families told the court there were "opportunities galore" to identify that the three men were plotting an attack.
Xavier Thomas, 45, Christine Archibald, 30, Sara Zelenak, 21, Sebastien Belanger, 36, James McMullan, 32, Kirsty Boden, 28, Alexandre Pigeard, 26, and Ignacio Echeverria, 39, were all killed in the attack.
The inquests continue.