Ali Mahwood-Awad Irsan was found guilty of capital murder last month in the 2012 fatal shootings of his son-in-law, Coty Beavers, and his daughter's friend, Gelareh Bagherzadeh.

The Harris County jury deliberated for just 35 minutes on Tuesday - after five weeks of testimony - before reaching the verdict in Houston.

The same jury deliberated about nine hours before sentencing him to death for the killings, which occurred about 11 months apart.

When the jury's punishment verdict was announced, Irsan slumped slightly while family members of the victims wiped tears from their eyes.

Prosecutors alleged that Irsan, a 60-year-old conservative Muslim, became enraged after his daughter married Beavers, a 28-year-old Christian, and converted to Christianity.

Investigators said Bagherzadeh had encouraged her friend to marry Beavers.

Irsan's wife, Shmou Alrawabdeh, testified at trial that her husband tried to "clean his honour" with the killings. She told jurors her husband also intended to kill their daughter.

Bagherzadeh was targeted first. Police said Irsan, his wife and their son, Nasim, followed the 30-year-old Bagherzadeh to her parents' home in January 2012, and that Nasim Irsan shot her in her car. Nasim Irsan is awaiting trial on a capital murder charge.

Beavers and Nesreen Irsan were married in in July 2012.

That November, Ali Mahwood-Awad Irsan slipped into Beavers' unlocked apartment near Houston, waited for his daughter to leave for work, then shot his son-in-law, according to Alrawabdeh, who testified in her husband's trial as part of a plea agreement with prosecutors.

At trial, Irsan told jurors he wasn't involved in the deaths but acknowledged his daughter had caused his family pain by marrying Beavers.

Australian Associated Press