Injured worker can collect workers’ comp and sue

Q. One of our employees badly hurt himself when he tripped over some equipment that had been left by the building maintenance. Workers’ comp covered his medical bills and lost wages, but he also sued the building for negligence. Now we have a letter from the office building demanding that we defend them in the lawsuit and telling us we are responsible. How can that be? I thought we were protected by workers’ comp from any suit involving an employee injury?