Write a summary of the article BlackBerry Messenger Plays a Starring Role in a Brazil Corruption Scandal.

Brazilian prosecutors have relied heavily on confessions from company insiders to expose an alleged graft ring at Petróleo Brasileiro SAPBR +1.47%.

But there was another crucial ingredient: thousands of instant messages that alleged wrongdoers sent each other via BlackBerry BBRY -0.21% Messenger, or BBM.

Those communications helped local authorities make connections between suspected money launderers and their alleged clients, including Brazilian lawmakers and construction contractors, officials said.

The digital sleuthing has led to criminal charges against more than 100 people as well as the investigation of close to 50 Brazilian politicians as part of a probe dubbed Operation Car Wash. The scandal has rocked Brazil's political establishment and the oil giant, known as Petrobras.

"This was one of the reasons for the success of the case," said federal prosecutor Deltan Dallagnol, referring to the BBMs.

Aided by search warrants and the cooperation of BlackBerry, the Canadian company that operates the instant-messaging system, prosecutors were able to review thousands of BBMs, according to Brazilian police and prosecutors.

Transcripts of messages, reviewed by The Wall Street Journal, were often mundane. Alleged participants shared jokes, sent inspirational quotes and swapped photos of their dogs. But the chatter proved useful for Brazilian authorities in identifying suspects and establishing relationships between them.

Some talked business. In one series of BBM exchanges, two men arranged meetings, cash pickups and bank transfers for an unspecified deal. They also bragged a bit about their flourishing partnership.

"We will dominate this country," wrote a man nicknamed LA.

"God willing, we will," a man nicknamed Primo responded. "Because we are good."

"LA" is Luiz Argôlo, a former congressman, according to prosecutors. Brazil's Supreme Court in March authorized federal prosecutors to investigate him and 47 other politicians suspected of taking bribes.

Authorities didn't have much trouble identifying Mr. Argôlo. He posted his home address in one of the messages, police say.

Mr. Argôlo, who has denied wrongdoing, couldn't be reached for comment.

His BBM correspondent, "Primo," is Alberto Youssef, who has confessed to money laundering as part of a plea deal in the Petrobras case. Mr. Youseff has denied other charges leveled against him-corruption and conspiracy-according to his lawyer, Antonio Figueiredo Basto. Mr. Basto confirmed that his client used the name Primo in BBM messages.

Mr. Youssef is currently in jail in southern Brazil awaiting sentencing for his role in Operation Car Wash. Last September, he was sentenced to four years and four months for violating terms of a plea agreement in a previous money-laundering case.

Prosecutors allege that executives from some of Brazil's largest construction companies conspired with Petrobras insiders to skim hundreds of millions of dollars from the state-controlled oil giant through inflated contracts, channeling some of the money to a slush fund that enriched politicians and secretly financed campaigns.

Four former Petrobras executives and at least 23 construction executives have been charged with crimes such as corruption and money laundering. Lawyers representing those companies have either denied involvement, said they were cooperating with investigations or didn't return requests for comment. So far, there have been no trials or convictions.

Defense lawyers are working on a number of strategies to extricate their clients. Attorney Edward Carvalho, who represents five executives from construction firm OAS SA and the company itself, maintains that Brazilian police didn't use the proper channels to gain access to the BBMs from BlackBerry, and he wants them thrown out. His clients, charged with crimes such as money laundering and corruption, have denied wrongdoing.

A judge earlier this year rebuffed Mr. Carvalho's first effort to get the messages tossed. He is preparing an appeal.

Brazilian police say they obtained the messages lawfully. BlackBerry declined to comment, pointing instead to a policy position on its website, which says the company balances any requests from legal authorities for "lawful access assistance" against "our priority of maintaining privacy rights of our users."

Transcripts of the BBMs point to familiarity, even friendships, among some of those accused in the case.

On New Year's Eve of 2013, Mr. Youssef sent this holiday greeting: "My friend Ricardo. In 2014 may you and your family realize all of your desires and projects with peace, happiness, prosperity. Yours, Primo."

The recipient was Ricardo Pessoa, chief executive of construction company UTC Engenharia SA, who is currently jailed on charges of money laundering and corruption. Mr. Pessoa has denied wrongdoing, said his lawyer, Alberto Toron.

Mr. Pessoa responded to Mr. Youssef a minute before midnight.

"My friend Primo. I wanted to thank you for the partnership and loyalty. I wish for you and your family great success and much happiness and health in 2014. Big hug. Ricardo."