But investigators have found themselves butting up against a door sealed firmly shut.
The witness with the memorable moniker, "Mr. Sidewalk," proudly shared his rags-to-riches immigrant success story. He apologized for any mistakes he might make speaking French, and explained that he'd picked up the language on the street.
When it came to the Mafia, though, Nicolo Milioto had little to say Monday.
The elderly, retired construction boss offered an explanation for those stacks of cash he was repeatedly seen handing to the since-killed Mafia don Nicolo Rizzuto, in a police surveillance video: It was for Sicilian community fundraising events.
Why, then, would the old Mafioso have been seen stuffing the cash into his socks? To keep it safe, Milioto replied. Women might stuff money into their bras, he said, and men might use socks.
Was he even aware that Nicolo Rizzuto was one of North America's most notorious Mobsters? Nothing more than what was written in the newspapers, Milioto replied.
He said he'd known Rizzuto his whole life, ever since he was a boy in the Sicilian town of Cattolica Eraclea. As for how Rizzuto had made his fortune, Milioto said, he didn't ask any questions.
"For me, he was a family father. He was a good person," Milioto said of the Mafia family patriach, who was gunned down in his kitchen in 2010.
"I knew what the newspapers said. That didn't affect me... For me, he was a good person."
The exchanges between commission chief counsel Sonia LeBel and Milioto often turned testy. She squinted in disbelief at his answers and her tone rose while he professed ignorance of the Rizzutos' business dealings.
When asked again how Rizzuto had made his money, he snapped: "I'm not his accountant."
The elderly construction boss was unknown to the public before the inquiry began but he has become a major figure in its hearings.
He has been described as a central hub in a wheel of corruption — as the key link connecting the Mafia, a major municipal political party and certain parts of the construction industry.
Police said he'd been spotted 236 times at the famous Mafia hangout in east-end Montreal, the now-shuttered Cafe Consenza.
He explained Monday that he often wandered in because his butcher shop happened to be next door.
He said would sometimes play cards with the elderly Rizzuto and, when asked what they discussed, he replied: "We talked about cards."
On the subject of those visits where he was seen bringing cash to the Rizzuto patriarch, Milioto talked about community fundraisers. He said the only time he ever brought money on behalf of another construction boss was for Lino Zambito.
Zambito happens to be the witness who first described Milioto's pivotal role in an alleged cartel.
Milioto said he didn't know what Zambito's payment would have been for — but he assumed it was because his fellow construction boss had been having money problems and had sought help from the Rizzutos.
As for companies paying protection money to the Mob, inquiry chair France Charbonneau asked whether he was aware of the practice of so-called "pizzo."
"I come from Italy, madame, I know what a pizzo is," Milioto replied. But he said he'd never heard of it happening in Canada.
Milioto grew frustrated as inquiry counsel pressed him for names of people he knew from his hometown of Cattolica Eraclea.
A large number of Italian immigrants to Canada come from that tiny town and, Milioto said, it would be unfair to start naming people at random.
"Why would I repeat the names of people who have nothing to do with the commission?" Milioto said.
"Give me the names, and I'll discuss them."
He eventually got more specific, and allowed that other construction bosses whose names had come up at the inquiry did in fact have roots in that town.
In recent months, Milioto has been described by witnesses as a central figure in the creation of a construction cartel.
He allegedly collected money from construction bosses and handed a share to the Mafia and to the now-crumbling Union Montreal party, which until recently had run the city for a decade.
One witness even said Milioto threatened to bury him in one of his sidewalks when he interfered in the cartel's operations.
Milioto was far less menacing as he took to the stand Monday. Upon taking the oath, he apologized for any mistakes he might make when speaking French, and explained that he'd picked up the language "in the street."
He said he came to Canada at age 18, and after working in the construction industry for years had managed to start his own company.
He described how, from humble roots, he built a successful career that saw him work in textiles and buy a grocery store before moving into construction.
He said he scrimped and saved to create that company, Mivela Construction, in 1989.
And when he owned that company, he said his working day would begin in the office at 5 a.m.
Milioto, who headed Mivela Construction until 2012, a firm that specialized in sidewalks, was also a key figure in a story recounted by witness Martin Dumont.
During that encounter, the ex-municipal employee recalled that he had mangled Milioto's name. In response, he said the boss told him, "You can call me Mr. Sidewalk."
During a later encounter in 2007, Dumont questioned a city engineer about why it was going to cost $100,000 more for a sidewalk project than a similar one six months earlier.
Dumont alleged that Milioto offered a brusque response while visiting a borough office where Dumont was an aide to a local mayor.
"You know, Martin, my sidewalk foundations are thick and deep," Dumont said Milioto told him.
"You don't want to end up in my sidewalk foundations."
Dumont told the inquiry that he had never told anyone about the conversation and became emotional when recounting it years later on the stand.
But Dumont's credibility has been under attack, with subsequent testimony calling into question the accuracy of some of his claims. He was even heard telling investigators that he'd made up one of the anecdotes during his testimony.
Milioto's testimony continues Tuesday.