Denver, Colorado (CNN) -- The father and uncle of Najibullah Zazi -- who is accused of conspiring to use weapons of mass destruction in a terrorist attack -- poured chemicals down a drain and shredded containers in an effort to destroy evidence, according to a source close to the investigation.
The source tells CNN that an FBI Evidence Response Team discovered chemical residue in pipes at a property in the Denver area, and found that attempts had been made to shred the containers that held the chemicals -- which included hydrogen peroxide, an ingredient in some forms of high explosives.
Najibullah Zazi's father, Mohammed Wali Zazi, was initially charged with lying to investigators, but last week a federal grand jury in New York charged him with conspiracy to obstruct justice.
The indictment said that he "did knowingly and intentionally conspire to corruptly alter, destroy, mutilate and conceal objects, to wit glasses, masks, liquid chemicals and containers," with the intent of impeding a grand jury investigation.
Najibullah Zazi's uncle, Naqib Jaji, was indicted on a single felony charge and was arraigned in a sealed proceeding in Brooklyn, New York, two weeks ago. The charge wasn't specified, but the source tells CNN the uncle had participated in the attempt to dispose of evidence.
Federal officials have said that the conspiracy involving the younger Zazi represents the single most serious terror plot since 9/11, and that the investigation is intense and ongoing.
The indictment against Najibullah Zazi, 24, alleged that he twice checked into a motel in the Denver area -- in August and September -- and was experimenting with heating chemicals that could create a bomb. The indictment said chemical residues had been recovered from a vent in the motel room.
CNN obtained closed-circuit video from beauty parlors in the same area purportedly showing Zazi buying large quantities of chemicals that can be used to make bombs. The indictment against the younger Zazi says he also was in contact with other people as he experimented with the chemicals and tried to make the high explosive TATP.
Authorities say Zazi planned to be in New York with the intent of using an explosive device around September 11, 2009. The alleged plot, which came to light after raids in New York in mid-September, may have been targeting a major transportation center, sources told CNN.
He has pleaded not guilty to one terrorism conspiracy charge.
The uncle told CNN in Denver last fall that he and his wife had appeared before the grand jury investigating the alleged conspiracy involving his nephew.
He blamed his nephew, who had lived with him for several months, for dragging his family into the national spotlight. The uncle said he had a respectable job, young children and a good home, but he felt obliged to take the younger Zazi in because he was related to his wife.
Authorities have indicated to CNN that the younger Zazi had been under surveillance for months before his September arrest in Colorado. Agents had followed him as he drove from Denver to New York.
Two other men have since been indicted by a federal grand jury in New York in connection with the case. Both were arrested last month.
Zarein Ahmedzay, a New York taxi driver, is accused of lying to FBI special agents about locations he visited during a trip to Pakistan and Afghanistan and about discussions he had with an unidentified man about military training in Pakistan in 2008. He has pleaded not guilty.
Adis Medunjanin, 25, originally from Bosnia, has pleaded not guilty to charges of conspiracy to commit murder in a foreign country and receiving military-type training from al Qaeda.