Kautilya's Arthashastra 9.6.53

9.6.53

tīkṣṇam utsāhinaṃ vyasaninaṃ sthitaśatruṃ vā gūḍhapuruṣāḥ śastrāgnirasādibhiḥ sādhayeyuḥ, saukaryato vā teṣām anyatamaḥ //

Spies under concealment may, without the help of a fiery spy, murder by means of weapons, poison or other things a fortified enemy who is of mean character or who is under troubles; any one of hidden spies may do the work when it is found easy; or a fiery spy alone may do the work by means of weapons, poison or fire; for a fiery spy can do what others require all the necessary aids to do.