No motive has been established yet.

きょう未明、相模原市の障害者施設で19人が心肺停止、20人が重傷、その他複数人が負傷した殺傷事件が発生しました。

Japan Sagamihara knife attack: At least 19 dead, reports say

At least 19 people have been killed in a knife attack at a residential care centre for the disabled in the Japanese city of Sagamihara.

The attack happened in the early hours of Tuesday. Another 45 people were reportedly injured, 18 seriously.

Local police have arrested a male suspect after he turned himself in.

The man is said to be aged 26, and is a former employee at the the Tsukui Yamayuri Garden facility, 40km (25 miles) south-west of Tokyo.

He reportedly went into a police station and said "I did it". No motive has been established yet.

Details are still emerging about the incident, but the death toll makes it the worst mass stabbing in Japan in recent years.

In 2008, a man drove a truck into a packed shopping district at Akihabara in Tokyo, before climbing out and randomly stabbing people. Seven people died.

The attack occurred on the same date that a man with a history of mental illness stabbed eight children to death at an Osaka primary school in 2001.

http://www.bbc.com/news/world-asia-36890655

記事の中に"No motive has been established yet."という文があり、これは「犯罪の動機がまだ解明されていない。」などと解釈することができます。英語では"establish"を用いて「動機を打ち立てる」というような考え方をするようです。

確認のために、LDOCEを引いてみたところ、"to find out facts that will prove that something is true"とありました。

現在は取り調べにより動機が明らかになり、施設を首になった腹いせにやったとのことです。これから詳しい捜査が行われます。
(Blue Sky)