A member of a polio-vaccination team and two police officers who were escorting it were shot dead in northwestern Pakistan, police told RFE/RL.
The attack took place in the Datta Khel area of Pakistan's North Waziristan tribal district on June 28. A child was also wounded during the incident, police said.
There was no immediate claim of responsibility for the attack.
Militants in Pakistan often target polio-vaccination teams and police details assigned to protect them, falsely claiming the vaccination campaigns are a Western conspiracy to sterilize children.
Pakistan launched a nationwide anti-polio vaccination drive on June 27 to inoculate 12.6 million children under the age of five.
Pakistan regularly launches anti-polio campaigns in an effort to eradicate the highly infectious disease.
The latest campaign was launched after a polio case was detected in North Waziristan on June 25.
Eleven people were detected in Pakistan as being infected with the virus this year compared to just one case last year. All cases were reported from North Waziristan.
Pakistan had never gone as long as a year without a new polio case since eradication efforts began decades ago.
Pakistan and neighboring Afghanistan are the only two countries in the world still listed as "affected" by the wild poliovirus, which is otherwise classified as eradicated by the World Health Organization.
Eradication efforts in both countries are hampered by local skepticism and deadly militant attacks on eradication teams, especially in rural areas.