GENEVA, June 1 (Reuters) - A fire that devastated Geneva's largest synagogue last week was caused by arson, the investigating magistrate said on Friday.

Investigators had believed an electrical fault might have been to blame for the pre-dawn blaze in the empty building, which injured no one, but had now established it was started deliberately.

Magistrate Michel Graber told Swiss news agency SDA that investigators were seeking a DNA analysis from a cigarette butt found at the scene.

The May 24 fire largely destroyed the Hekhal Haness synagogue, built in the 1970s.

Wealthy Switzerland is known for public order and tolerance but has become increasingly the focus of religious tension, particularly targeting its Muslim minority.

Anti-Semitic acts that become public are rare, but in June 2001 an Israeli rabbi was shot dead on the streets of Zurich.

The fire took place during the Jewish holiday of Shavuot, which marks the giving of the Torah, the Jewish holy book, by God to Moses on Mount Sinai over 3,000 years ago.

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