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.

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.