Four people allegedly tied to a north Minneapolis street gang embroiled in a turf war are being indicted with conspiracy built around their use of illegal firearms in a series of shootings last year, according to federal charges.

In a novel strategy aimed at building federal cases against Twin Cities street gangs, investigators went after guns used by Tre Tre Crips members during drug deals or feuds with rival factions.

Among those indicted was a "straw purchaser" who — unlike her co-conspirators — was able to buy firearms because she had no prior felony conviction.

Charles Bickham, aka "Tall Cuz," and Oprah Andrea Latrice Ann Gray made their first appearances and pleaded not guilty in U.S. District Court in Minneapolis on Tuesday. Trudale Raymond Williams, identified in a search warrant application unsealed this week, is expected in court Thursday, and a fourth unnamed defendant is still a fugitive. Attorneys for Bickham and Gray declined to comment on Tuesday.

Gray purchased two semi-automatic pistols from a Bill's Gun Shop and Range in May and June 2016. In one case, investigators linked one of the weapons to a shooting in north Minneapolis committed just a day after the June purchase, according to charges.

Authorities from the U.S. Marshal's Service, U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and Minneapolis police investigated the case. A confidential informant last fall tipped off investigators about Williams possessing "a large amount of marijuana and multiple firearms" and confirmed him as an active member of the Tre Tre Crips, according to the search warrant. At the time, Williams was on probation for federal weapons charges.

Gang members acquired firearms through theft, burglaries and using "straw purchasers" with no felony convictions to lawfully buy guns for them, according to charges. Once gang members acquired the guns, they shared them within their circle. During the roughly two-year span of the investigation, the Tre Tre Crips were involved in a gang war with several rival gangs, including the Stick Up Boys and the T-Block gang.

According to the search warrant, investigators relied on numerous informants and used surveillance devices to gather information about incoming and outgoing phone calls. Authorities seized an unspecified amount of illegal firearms and narcotics throughout the investigation and said the group was behind the sale of large quantities of cocaine, heroin and marijuana in Minneapolis.

The federal charges also include multiple separate counts of illegally possessing firearms and ammunition and two counts against Gray for making false statements when purchasing the two pistols last year. Bickham was prohibited from possessing a gun by previous drug and weapons charges in Hennepin County and the U.S. District of Minnesota.