The US attorney’s office has joined the investigation into the hepatitis C outbreak at Exeter Hospital’s cardiac catheterization lab. State public health officials have said they suspect a lab employee’s misuse of drugs led to the outbreak. A hospital worker and 19 of the lab’s patients have tested positive for the liver-destroying disease since the investigation began last month. US Attorney John Kacavas said Tuesday that his office brought in personnel from the Food and Drug Administration and FBI to determine whether the outbreak is a criminal or civil matter. A Concord lawyer preparing a class-action lawsuit against the hospital said he now has 23 plaintiffs.