The Seattle teen was visiting friends in Lake Stevens on June 1 when she was struck in the neck by a bullet.
Detectives have been working around the clock to find the shooter.
Word of the arrest came Friday with a Twitter post from the city of Lake Stevens just before 9 p.m. That was followed by a brief press release from the sheriff's office confirming the arrest and offering little else.
Sheriff's spokeswoman Shari Ireton said that the suspect, 26, was arrested without incident at his Marysville-area home. Detectives weren't immediately available for more details.
The man was expected to be booked into county jail sometime Friday night for investigation of first-degree murder.
Police reported receiving the call about Molly's shooting at 11:18 p.m. June 1 near the intersection of S. Lake Stevens Road and S. Davies Road. Several of Molly's classmates were with her when she was shot.
There were other incidents of gunfire after Molly was killed. A shots-fired call was reported around 12:45 a.m. near the 15000 block of 61st Street NE, northeast of the fatal shooting. Another occurred around 2:30 a.m. near the 5400 block of 105th Street NE in north Marysville. No injuries were reported.
Police in Marysville also received a report of shots being fired at a home and two cars in the 10500 block of 56th Drive NE around 2 a.m. June 2. No one was hurt in that incident.
Few details about the shooting have been released. Then last week the sheriff's office took the unusual step of of phoning thousands of people living in Snohomish County in hopes of generating leads. The recorded message, sent as a reverse 911 call, went to nearly 4,000 phones.
Detectives were asking people to step forward if they had video of vehicles driving in the area during the hours before and after the shooting. Specifically detectives asked for information about any black passenger vehicles that had front, passenger side damage.
It was unclear Friday if those calls helped detectives identify a suspect.
Since the Seattle's teen's death there has been an outpouring of support by the Lake Stevens community and those who knew the girl. Reward money was raised to entice anyone with information about the shooting to step forward. Community members and Molly's friends held a march in her honor on June 17.
Molly was the youngest of three children. She was finishing her freshman year at Bishop Blanchet High School. She just turned 15 the day before she was killed.
She was described as a giving and compassionate girl.
"We loved Molly so very much," her mother, Susan Arksey, said. "She was a great girl, she loved her family, her friends, her school and she loved life. She loved her sister, her brother and her parents."
"… She had a generous spirit," Arksey added.
An example is the organization called "Mother's Helper," which Molly and her mom started. It helps mothers with babies who have just arrived in domestic violence shelters in Seattle. Molly came up with the name of the organization, which provides diapers, formula and clothing for babies, her family said.
On Friday hours before the arrest, Molly's mom wrote on Facebook about receiving a package from her daughter's high school earlier in the day. Inside was a letter Molly wrote to her classmates as a part of a school assignment. It also contained her last report card -- straight As. Molly was nicknamed "4.0" for her perfect grade-point average.
In her letter, Molly wrote about the importance of tolerance and kindness.
"If we all get to know each other now it will make our lives together that much better. Now I am not asking for everyone to be great friends but the least we could do is respect one another. This is like how Jesus says to treat others how you would like to be treated. If we do that then we would no longer hurt each other. Another way to say that is put yourself in their shoes. We need to know that everyone has different things they are dealing with. It is very often that I forget the world does not revolve around me. I admit that I get caught up in my own life and forget other people have troubles also. When I remember that I am able to be more understanding of others. If we all step back for a second and remember we all have our troubles we will be able to respect each other more easily."
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