D.C. Mayor Vincent Gray is shown in January. (Jim Cole/AP)

A second key member of Vincent C. Gray’s 2010 mayoral campaign has been charged in a broadening campaign finance investigation that is targeting high-level District officials.

The charges came Wednesday, a day after a close Gray friend and campaign staffer pleaded guilty to a federal charge of obstructing justice and to three D.C. election law violations that involved secret payments to a minor candidate in the hopes of bolstering his boss’s election prospects.

The new charges and Tuesday’s plea deal, which came with an extensive cooperation clause, signal that federal authorities are hoping that the two lower-level aides can help them build cases against others who worked on Gray’s campaign.

Federal authorities also are scrutinizing a prominent D.C. contractor, Jeffrey Thompson, and his extensive ties to the mayor, his administration and D.C. Council members. Thompson’s house and offices were raided by federal agents in March, and authorities have served federal grand jury subpoenas on a number of D.C. Council members seeking records of their dealings with the contractor.

Wednesday’s charges involved Howard Brooks, 64, a consultant who worked on the Gray campaign’s treasury and financial teams. Brooks was accused of lying to FBI agents when he denied giving cash or money orders to a minor mayoral candidate in the 2010 campaign. In fact, prosecutors have alleged, he gave that candidate, Sulaimon Brown, hundreds of dollars in the hopes of keeping him in the race to continue assailing incumbent Mayor Adrian Fenty in the Democratic primary.

Gray, then chairman of the D.C. Council, defeated Fenty in the tightly contested primary and later cruised to a general-election victory. Hours after Wednesday’s charges were made public, the mayor deflected questions, saying he could not comment about an “ongoing pending investigation.”

“I’m focused on the city at this stage, and I’m going to do everything I possibly can,” he said. “I’m focused on the job I was elected to do, and I’m working all day, every day.”

Brooks, who was paid $44,000 by Gray’s campaign, was charged in a “criminal information” with one felony count of making a false statement to FBI agents. A criminal information can be filed only with a defendant’s consent and generally signals a plea deal has been reached. Brooks is scheduled to appear at a plea hearing Thursday afternoon in the District’s federal court. He could not be reached for comment; his attorney, Glenn Ivey, declined to comment until after Thursday’s proceeding.

It will be the second such hearing this week. Thomas W. Gore, the Gray campaign’s former assistant treasurer and a longtime confidant of the mayor’s, pleaded guilty to related charges Tuesday.

In court papers, prosecutors alleged that Brooks and Gore discussed a plan in June 2010 that would funnel illicit donations of excess and “unattributed” cash campaign donations to Brown to keep him in the Democratic primary. Gore used Gray’s campaign money to obtain at least five money orders worth $660. After ensuring the orders were signed in the names of his family members or one of their friends, Brooks passed them along to Brown, according to court records filed in Gore’s case.

Like Gore, who admitted to destroying a notebook that documented the payoffs and then lying about it to federal agents, Brooks appears to have gotten in serious trouble for trying to cover up the initial crime. Prosecutors say he lied to agents when he denied giving Brown “cash, money orders, or other types of payments.”

The Washington Post first reported the payments to Brown in March 2011. Brown also said he was promised a city job, an allegation that Gray denies. Brown worked briefly for the D.C. Department of Health Care Finance when Gray first took office.

Although the amount of money involved in the payoffs was relatively small, the District’s top federal prosecutor, Ronald C. Machen Jr., said the criminal charges were significant because “these cases involve attempts to hide the truth from the voters of D.C. That goes to the integrity of the process.”

Machen, the District’s U.S. attorney, declined to discuss other details of the “ongoing investigation.”

But former federal prosecutors said the charges — and Gore’s cooperation agreement — indicate that authorities think they can apply pressure to relatively low-level campaign workers to help them build cases against more significant figures, perhaps even the mayor. The charges also represent a milestone in the 14-month investigation because it is the first time authorities have publicly spelled out some of their evidence of illicit activities in the Gray campaign, they said.

“They are squeezing as much as they can out of the Sulaimon case, and they would not be executing search warrants unless they had more information indicating there were more wide-ranging improprieties,” said Michael Volkov, a former federal prosecutor. “It’s clear this case is not just about Sulaimon Brown’s allegations. This is bigger and about building a case against Gray or those close to him.”

Sources familiar with the investigation say it has broadened beyond the original allegations involving payments to Brown. In March, federal agents raided Thompson's home and offices. Thompson, 57, is a self-made businessman and philanthropist who also flooded local and federal races with tens of thousands of dollars in contributions. He has personally given more than $200,000 in donations to federal committees and campaigns.

D.C. Chartered Health Plan, a managed care firm from which he stepped down as chairman last month, is the city’s single largest vendor with a $322 million annual contract. Thompson, Cobb, Bazilio & Associates, one of the largest black-owned firms in the country, also does millions of dollars of auditing work for the city.

In the District, Thompson was known as a go-to bundler, helping the campaigns of former mayors Anthony A. Williams and Adrian M. Fenty, as well as Gray and several council members and unsuccessful candidates.

In the 2010 District elections, records show, Thompson and his associates increasingly relied on making campaign contributions through money orders, a type of payment that election law experts say can facilitate donations made in others’ names.

Thompson could not be reached for comment. His attorney, Brendan Sullivan, declined to comment.

Separately, D.C. Council Chairman Kwame R. Brown (D) is the subject of a federal investigation delving into unreported transactions and other irregularities associated with his 2008 race for an at-large council seat.

Staff writer Mike DeBonis and researcher Lucy Shackelford contributed to this report.