(Recasts with higher death toll, adds quote)

TIJUANA, Mexico, Jan 15 (Reuters) - Gunmen killed eight people including three senior police officers and a 3-year-old boy in the Mexican border city of Tijuana, a week after the government beefed up security there against drug gangs.

Two of the officers were gunned down by suspected drug hitmen while driving near a mall in the city's southern La Mesa district. Another policeman was shot dead at home along with his wife and 9-year-old daughter, police said on Tuesday.

Another man, his wife and their 3-year-old son were shot dead while asleep in what police said was a case of mistaken identity.

Police believe the same gunmen were responsible for all eight killings.

Mexico sent hundreds of police and army reinforcements last week to Tijuana, just south of San Diego, following a rash of drug killings. Tijuana is the biggest city in Baja California, which was Mexico's most violent state in 2007 with more than 400 gangland-style murders.

"We know this is a war and we have to win it every day," state Gov. Jose Guadalupe Osuna told reporters.

President Felipe Calderon has been battling organized crime since he took office a year ago and has sent some 25,000 troops and federal police to the country's worst trouble spots.

On Monday alone, 17 people were found dead across the country in drug-related crimes, Mexican media reported. (Reporting by Lizbeth Diaz)

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