BELGRADE, Serbia (AP) — Serbian university students left piles of old school books outside the education ministry building on Friday as part of almost daily street protests demanding accountability over the collapse nearly two months ago of a concrete canopy that killed 15 people in the country's north.

Hundreds of Serbian students and others again blocked traffic for 15 minutes throughout the country as political tensions continue to simmer following last month's collapse of a concrete canopy that killed 15 people.

Prosecutors arrested 13 people over the tragedy, but much of the public is skeptical about the honesty of the investigation