The two companies are under investigation for colluding to manipulate market prices.

colluding 是动词 collude 的现在分词形式。collude 由前缀 col-（表示"共同"）和拉丁词根 ludere（"玩耍、欺骗"）组成，意思是"暗中勾结、密谋"，通常指为了不正当目的而秘密合作。