(CNN) — A little over a week ago, Margaret “Maggie” Mbitu was finishing her shift as a nurse in suburban Boston and getting ready for an upcoming girls’ trip to Las Vegas.

Now her stunned family is planning her funeral.

The first sign of alarm came last Tuesday, when the 31-year-old didn’t show up for work. The next evening, police made a gruesome discovery: her bloodied body, in a car inside a parking garage at Boston’s Logan Airport.

“I’m so angry, I’m still trying to process everything,” Ann Mbitu, her older sister, told CNN. “At 31, we’re not supposed to be planning her funeral. We’re supposed to be planning celebrations, birthdays, weddings, travel.”

Mbitu’s suspected killer is believed to have fled the country. The day before her body was found, Kevin Kangethe, who police describe as Mbitu’s boyfriend, boarded flights from Boston to Kenya, where he remains on the run. Authorities have obtained a warrant for his arrest.

Police haven’t divulged a possible motive, leaving Mbitu’s family and friends to grapple with unanswered questions – and their grief.

“Why her?” Ann Mbitu said, her voice cracking with emotion. “Nobody’s daughter deserves this, but why my sister? Why Maggie?”

Investigations into Mbitu’s killing have spanned two continents

Mbitu was last seen around 11 p.m. on Monday, October 30, while leaving her job in Halifax, about 35 miles south of Boston. A missing-person flyer said she was wearing black pants and a black cardigan over a gray shirt.

She was a no-show at her job the next day, which was uncommon for her. Her employer contacted Mbitu’s family, who notified the police and called nearby hospitals to check if she was a patient.

In a criminal complaint from the Massachusetts State Police and obtained by CNN affiliate WFXT, authorities say they “were led to Mbitu’s  boyfriend” after she went missing. Kangethe, 40, lived in Lowell, a suburb northwest of Boston.

License plate recognition (LPR) cameras had picked up the whereabouts of Kangethe’s Toyota SUV and it appeared consistent with the location of Mbitu’s phone, according to the criminal complaint.

Authorities tracked the vehicle from Lowell to the Logan Airport parking garage, where they found Mbitu dead in the front passenger seat with slash and stab wounds on her face and neck, police say.

Investigators learned that Kangethe had bought a plane ticket to Kenya the previous morning, state police said.

Surveillance footage showed him leaving the parking garage and entering an airport terminal, where he boarded a flight, police said.

Investigators have not revealed a motive for the killing, but it “was not a random act,” state police said in a statement. Police declined to comment further.

In the criminal complaint, police say that based on the evidence there is probable cause to believe that Kangethe committed murder. They say they are working with authorities in Kenya to track him down.

Resila Onyango, a spokesperson for Kenya’s National Police Service, did not respond to CNN’s requests for comment.

“We urge this suspect to turn himself in to authorities before he or anyone else gets hurt,” said Suffolk County District Attorney Kevin R. Hayden in a statement.

“We are making every effort possible to apprehend him and to begin the process of seeking justice for Margaret Mbitu and those mourning her tragic death.”

She was the youngest in a family of nurses

Mbitu was the youngest in a family of health care workers – her two older sisters and her mother are all nurses. The family came to the United States from Kenya in 2007 and Maggie Mbitu earned a nursing degree from Curry College in Milton, Massachusetts.

She had worked as a nurse for about seven years, most recently at BAMSI, a suburban Boston nonprofit that serves people with developmental disabilities and mental and behavioral challenges.

“She was warm, caring, and loved by everyone she worked with, both staff and the people she cared for in our group homes,” the nonprofit said in a statement on Facebook, adding it was mourning the loss of an “amazing” young  woman.

Mbitu was soft-spoken with a hearty laugh, her sister said.  She loved to travel – this year alone, she’d been to Costa Rica, Cabo San Lucas, Saint Martin and Thailand, her sister said.

“She was our little sister, but she behaved like a big sister to all of us,” she added. “Always trying to help everybody, making sure everyone’s OK, checking in on everyone.”

Her younger sister also was the glue that held her group of far-flung friends and relatives together, Ann Mbitu said.

“She was so level-headed. Work was a big part of her life, but when she wasn’t working, she tried to balance her life by spending time with everybody, bringing everyone together,” her sister says. “She was the type of person who if you needed a plus-one for a trip, you picked her because you knew she’d be there.”

Mbitu said she hopes Kangethe is arrested and returned to the Boston area to face justice. Kenya has an extradition treaty with the United States.

Meanwhile, her family has set up a GoFundMe to help with funeral arrangements and related expenses.

And they’re still struggling to accept the unthinkable.

“We were hoping that she had just disconnected and needed some time to herself,” Mbitu said. “Though that would still be unusual for her, we were praying that would be the case. We were not prepared for this outcome.”

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