Best Investigation movies of 1971

Actors : Helena Rojo, Jorge Luke, Cameron Mitchell, Isela Vega,
Directors : Alberto Mariscal,

A vengeful widow hires a professional killer to train her son so that he can hunt down and kill the men who murdered her husband. The quest for revenge soon becomes an obsession.

Actors : Sung-il Shin, Jeong-im Nam,
Directors : Woo-yeol Jeon,

Sang-hyeon is accused of murder and smuggling by his former teacher who is now a police investigator.

Actors : Donn Greer, Sebastian Brook, Gray Daniels, Mady Maguire,
Directors : Eric Jeffrey Haims,

A man commits a series of murders due to his dual personalities and his zodiac birth sign.

Actors : Anjanette Comer, Joanne Dainton, Keith Barron, Chad Everett,
Directors : Sidney Hayers,

Insurance investigator must find out who is setting fires. Along the way he meets and works with a beautiful newspaper reporter and falls in love.

Actors : Alberto de Mendoza, Janine Reynaud, Erika Blanc, George Ardisson,
Directors : Bitto Albertini,

A gangster seeks those who killed his brother. A series of murders follow.

Actors : Miranda Campa, José Luis López Vázquez, Carroll Baker, Michael Craig,
Directors : Eugenio Martín,

A wealthy Englishman finds his third wife dead. After the police discover that his first 2 wives had also died suddenly, an investigation is lead. Meanwhile, a new neighbor moves in and she seems to become very interested in Arthur.