A small explosive device carried by a makeshift drone blew up Sunday at the headquarters of Russia’s Black Sea Fleet on the Crimean Peninsula, wounding six people and prompting the cancellation of ceremonies there honoring Russia’s navy, authorities said.

Meanwhile, one of Ukraine’s richest men, a grain merchant, was killed in what Ukrainian authorities said was a carefully targeted Russian missile strike on his home.

There was no immediate claim of responsibility for the drone explosion in a courtyard at the naval headquarters in the city of Sevastopol. But the seemingly improvised, small-scale nature of the attack raised the possibility that it was the work of Ukrainian insurgents trying to drive out Russian forces.

A Russian lawmaker from Crimea, Olga Kovitidi, told Russian state news agency RIA-Novosti that the drone was launched from Sevastopol itself. She said the incident was being treated as a terrorist act, the news agency said.

Crimean authorities raised the terrorism threat level for the region to “yellow,” the second-highest tier.

Sevastopol, which was seized along with the rest of Crimea from Ukraine by Russia in 2014, is about 170 kilometers (100 miles) south of the Ukrainian mainland. Russian forces control much of the mainland along the Black Sea.