A 17-year old boy has been killed and a woman in her 40s injured in a bomb blast in Austin, Texas on Monday, 10 days after a man died in a similar attack.

All three victims are African American. In a second attack on Monday, a 75-year old Hispanic woman sustained life-threatening injuries.

Police are linking all three blasts. “We cannot rule out that hate crime is at the core of this, but we're not saying that that's the cause as well," said Brian Manley, Austin’s police chief.

The two blasts on Monday came as an estimated half million people gathered for an entertainment and media festival, South by Southwest.

In the first attack which took place at breakfast time a package, which had been left on the front doorstep, exploded after being brought into the kitchen, killing the teenage boy and injuring a woman.

Five hours later, an elderly woman was badly injured when opening a package which was left outside her home. She is said to be in a critical condition.