Seoul Metro CEO Chang Jung-woo resigned Friday over the subway collision that injured more than 246 people last week.

A train rear-ended another at Sangwangsimni Station on Seoul’s busiest subway line, Line 2, on May 2. The operator said the accident was caused by an apparent mechanical malfunction of its signal system. 

Seoul Mayor Park Won-soon said in a briefing that the CEO offered to resign early in the morning.

“(Those involved) will be held responsible,” Park said. “For now, it is important to find preventive measures and fill the administrative vacuum.”

Chung’s resignation came a day after another subway accident occurred on Line 1 at around 2:35 p.m. The operator said the train backed for about 300 meters after receiving the wrong signal from the monitor.

By Suk Gee-hyun (monicasuk@heraldcorp.com)