D.C. Police Chief Cathy L. Lanier said Thursday that the search for a missing 8-year-old girl has turned into a “recovery mission,” signaling that law enforcement authorities do not believe that Relisha Rudd will be found alive.

The chief said at an afternoon news conference that Relisha was last seen with the man who police say abducted her March 1. The following day, she said, Kahlil Malik Tatum bought a box of black 42-gallon contractor trash bags and was seen spending time in Kenilworth Park and Aquatic Gardens, in Northeast Washington near the Maryland line.

“We have not given up on hope that we may find Relisha alive,” Lanier said as officers and federal agents with cadaver dogs combed through the 700-acre park along the Anacostia River during a 10-hour­ search.

“But we cannot ignore the possibility that he may have killed her,” Lanier added, as city police officers and cadets in neon-green vests combed the woods section by section, walking slowly and scanning the ground. The search will resume Friday, authorities said.

In describing the chronology of events, the chief upended a timeline that District officials provided Wednesday regarding Relisha’s whereabouts. School officials had said earlier that Relisha attended school March 5, and a person familiar with the investigation said that she was seen at the school March 7. Lanier said Thursday that there have been no confirmed sightings of Relisha after March 1.

People familiar with the case said Thursday that a D.C. Superior Court grand jury has been empaneled to investigate possible obstruction of justice charges against Relisha’s mother, 27-year-old Shamika Young. Police and other officials said Young allowed Tatum to care for Relisha starting Feb. 26 and has repeatedly assured people that her daughter is “in a safe place.”

Two police officials said that Young did not want to file a missing persons report when police got involved March 19. She told detectives and the news media that she had talked to Relisha by telephone as late as March 17, and she told police her daughter had accompanied Tatum, whom she described as “Dr. Tatum,” to a medical conference in Atlanta.

In a brief interview by phone Thursday night, Young said she has been cooperating with police and believes that she has been unfairly cast as a bad mother.

“It’s not my fault,” Young said. “I am tired of laying my head down to get some rest and I can’t reach out to grab my daughter.”

Tiffany Young, one of Relisha’s aunts, said she is growing less hopeful that her niece will be found alive. “My body just gets these cold chills like a cold wind passing through,” she said as she watched news of the search for her niece. Though she didn’t use the words “dead” or “death,” she said, “I am starting to take it as if it is that.”

Police said that after Relisha was no longer seen, Tatum continued to go to his janitorial job at the homeless shelter at the old D.C. General Hospital, where Young lives with her children, who include Relisha’s three younger brothers.

Police said Tatum quickly left the shelter March 19 as a child welfare and school social worker came to press him about the child’s mounting absences from Payne Elementary School. Relisha had missed more than 30 days of class by then, but many of the absences had been excused because the family produced the note from a “Dr. Tatum.” The social workers learned March 19 that Tatum was not a doctor.

Police said that Tatum fatally shot his wife in the head later that night or early the next morning in a motel room in Oxon Hill and that he has not been seen since. His wife’s body was found March 20. Lanier said Relisha was not with Tatum at the motel.

Tatum remains on the run with a $25,000 reward on his head, and alerts have been issued from Pennsylvania to Florida. Police are concentrating on Richmond and the Atlanta area. There is a $45,000 reward for Relisha’s safe return.

The new timeline offered by Lanier runs counter to the District’s earlier explanation of how school authorities handled Relisha’s­ case and indicates a longer delay than previously thought between when Relisha was last seen and when city social workers investigated her extended absences from school.

Earlier this week, District officials said they did not notify child welfare authorities about Relisha’s­ many absences because they believed they had been excused by a doctor. Beatriz “BB” Otero, the deputy mayor for health and human services, told reporters at a news conference Wednesday that Relisha had been seen in school March 5, described as her last full day of attendance. One person familiar with the investigation also had said that a teacher saw Relisha at the school March 7 and that the girl said she was ill and was staying with her grandmother.

But police could not confirm the March 5 and March 7 sightings after interviewing school employees, according to a person familiar with the investigation. Authorities double-checked and learned that information from a teacher and a front-desk clerk conflicted about whether Relisha was at school March 5.

And the teacher who initially remembered talking to Relisha on Friday, March 7, said the conversation could have occurred the previous Friday, Feb. 28. “There is an ongoing investigation about when she was where on what days, and as that investigation goes forward, the days may shift,” Pedro Ribeiro, a spokesman for Mayor Vincent C. Gray (D), said Thursday. “This is something continually in flux.”

Police would not provide details on why they say Relisha may be dead, other than saying they are acting on a tip.

The FBI has previously released video showing Tatum and Relisha walking down a hallway of a Holiday Inn Express in Northeast Washington on Feb. 26.

Lanier said police will continue to look for Relisha beyond the park. Officers searched near an apartment complex where Tatum’s family lived in the 1980s and Tatum’s apartment in the 2500 block of N Street SE, near the shelter. Documents filed in D.C. Superior Court show that police were looking for a gun and any items linked to Relisha.

The court papers say officers seized several items from Tatum’s apartment, including children’s clothing and shoes, mail, two passports, a photo of Relisha, two cellphones and a box for an iPhone 5, a computer tablet and $87 that was on a table.

Mike DeBonis, Keith L. Alexander, Ann E. Marimow , Clarence Williams and Jennifer Jenkins contributed to this report.

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