NEW YORK (AP) — Activist investment firm Starboard Value says it bought an 8.2 percent stake in armored car company Brink's, and it called the stock undervalued.

Starboard Value disclosed ownership of almost 4 million shares in a form filed with the Securities and Exchange Commission on Monday. According to FactSet, that would make Starboard the second-largest owner of Brink's Co. shares.

Starboard Value LP started buying shares of the Richmond, Virginia-based company March 10. It didn't disclose any specific problems with Brink's or its management, but said the shares were undervalued and an attractive investment opportunity.

Brink's Co. shares rose 32 cents to $30.61 on Monday. The stock began climbing in February after Brink's reported its fourth-quarter results and rose further after the company's first-quarter report on Friday. The shares are up 25 percent in 2015.