An Indian-born doctor accused of illegally owning high-explosive grenades and his wife face new federal charges accusing them of interfering with the investigation into their extensive weapons cache.

The seven-count indictment, unsealed Friday, accuses Dr. Randeep Mann and his wife, Sangeeta Mann, of attempting to hide from federal agents signed, blank checks. The indictment also accuses Sangeeta Mann, 48, of lying to a federal grand jury about removing the checks from her husband's medical office on his orders.

In a statement, the U.S. attorney's office in Little Rock said federal agents arrested Sangeeta Mann on Friday. She pleaded not guilty to the charges and faces a detention hearing Monday. Her husband, who also has pleaded not guilty to earlier charges, remains held without bond pending trial.

Prosecutors allege in the indictment that Sangeeta Mann told the grand jury investigating her 51-year-old husband that she took a set of checks from his Russellville office because she "thought it would be safer."

"The checks had not been removed from the clinic because she thought it would be safer but because she had been instructed by Randeep Mann to remove the items," the indictment reads.

A spokeswoman for the U.S. attorney's office declined to discuss what prosecutors believe the checks were used for.

Sangeeta Mann's lawyer, Cathleen V. Compton of Little Rock, did not return a call for comment Friday.

City workers taking a bathroom break in a wooded area near the Mann home in March stumbled across a plastic-wrapped canister containing the grenades. Federal agents immediately suspected Randeep Mann, who had shown investigators a grenade launcher a month earlier when they questioned him about an explosion that critically injured Dr. Trent Pierce, the chairman of the Arkansas State Medical Board. U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives agents have said they don't consider Mann a suspect in the February bombing that struck Pierce's car at his West Memphis home.

ATF agents armed with a search warrant later raided Mann's home and found about $1 million worth of machine guns spread out on the floors, inside closets and secured in safes. Agents say the search also turned up 45 practice grenade rounds, two launchers, a roll of plastic similar to what was used to wrap the canister found in the woods and other canisters that looked the same.

Initially, a court affidavit filed by an ATF agent to justify the doctor's arrest claimed 17 of the machine guns may have been illegally owned by Mann. However, the grand jury did not initially return an indictment against Mann over the machine guns, only a single charge over hand grenades.

In the indictment unsealed Friday, federal prosecutors charged Mann with illegally possessing an automatic machine gun without official paperwork on the weapon. Mann also faces two new charges accusing him of owning an unregistered 12-gauge shotgun and the unregistered 7.62-mm machine gun.

Little Rock lawyer Blake Hendrix, who represents Mann, said his client would plead not guilty to the new charges. Hendrix said Mann, who had a federal license to sell machine guns, legally owned the weapons.