A federal judge in Illinois has rejected defense motions to suppress evidence against the man accused in the kidnapping and presumed killing of Yingying Zhang, a graduate student from China.

On Monday, US District Court Judge James Shadid denied six motions made by defense attorneys for the defendant, Brendt Christensen, 28.

The motions were made at pre-trial hearings last month in Peoria and Urbana, where Christensen's lawyers argued to against evidence obtained in the federal death-penalty case.

The evidenced included that gathered during an FBI search of Christensen's apartment and secret recordings of the suspect made by his girlfriend at the direction of federal agents, as well as statements Christensen made in the weeks leading up to his arrest.

According to the The News-Gazette in Champaign, Illinois, Christensen's then-wife, Michelle Zortman, testified at the hearings last month that she was startled and intimidated by agents who conducted a search before she agreed to it.

The FBI agents' testimony directly contradicted hers by describing an orderly search of her apartment the night of June 14 after they obtained consent.

"If Defendant's contentions and Ms. Zortman's testimony were accurate, they would indeed point toward involuntariness," the judge wrote in rulings made public Monday. "However, six FBI agents testified credibly that they entered the apartment at Defendant's invitation, requested that Ms. Zortman put on clothes, and asked her consent to search when only two agents remained in the apartment and no search was yet underway.

The judge wrote that "taking into account the totality of the circumstances, Ms. Zortman's consent to the search of the shared apartment was voluntary. Agents lawfully conducted the search, and evidence seized as a result of the search should not be excluded."

The judge also rejected a defense motion to make the death penalty unconstitutional, as well as a request to dismiss federal charges due to lack of jurisdiction.

"Both automobiles and cell phones qualify as instrumentalities of interstate commerce," he wrote, and therefore qualify the case for federal jurisdiction, even if they're only used within one state, adding that whether those items were used to commit the alleged crime is a matter for trial.

Wang Zhidong, a lawyer representing members of Zhang's family, said they "expressed their gratitude and respect to the rulings made by the federal judge".

"The judge's rulings are very important because they would directly affect the results of the trial," said Wang. "For example, if the judge had ruled that the case doesn't qualify for federal jurisdiction, the case would have been completely overturned."

"Plus, all the crucial evidence now can be used during the trial," Wang said. "Most of the efforts made by the defense are gone."

Zhang, 26, went missing in Urbana, Illinois, on June 9, 2017. She was last seen then entering Christensen's car near a bus stop on the campus of the University of Illinois at Urbana-Champaign (UIUC), where she was studying agriculture. Authorities say Zhang, who was from East China's Fujian province, is presumed dead; her body has never been found.

Christensen is due to stand trial in April in Peoria, Illinois.