The police have roped in the Federal Bureau of Investigation's hate-crime investigators to probe the killing of two Sikh restaurateurs in California, United States, while the reward being offered for any information leading to the assailants has been doubled.

The Richmond Police Department had offered a $10,000 reward for information leading 'to the arrest and conviction' of the killers of Ravinder Kalsi, 30, and his brother, 42-year-old Paramjit, who were shot dead a few minutes after closing their Sahib Indian Restaurant on the night of December 27.

An anonymous benefactor has now offered an additional $10,000, Gurman Bal, a friend and former roommate, was quoted as saying. Bal had earlier said he feared the killings could be a hate crime.

The two assailants walked up to the door, knocked to attract Ravinder's attention and began shooting when he opened the door. "The surveillance camera footage of the suspects has generated many calls and tips," Richmond police Detective Sgt. Mitch Peixoto said, "But so far, nothing has panned out".

The local police have also enlisted aid from FBI hate-crime investigators to probe the possibility of that motive, San Jose Mercury news reported.

The brothers, hailing from Punjab, ran the restaurant for five years, originally a sideline to their work renovating and selling residential properties. The police say they planned to sell the restaurant, but so far they can deduce no motive from the family finances.