A massive fire at a large residential construction site early Tuesday in West Oakland — the East Bay’s sixth major conflagration at a housing development in a little more than two years — has renewed concerns that a serial arsonist is targeting projects across the region. That suspicion was fueled by a second discovery Tuesday morning: a gas can, rag, match, hammer and the remnants of an intentionally set fire that burned itself out 10 blocks away at an under-construction apartment complex not far from the Oakland-Emeryville border. Oakland and nearby cities have been on high alert ever since video footage captured a hooded bicyclist sneaking past two security guards and a network of cameras to ignite a blaze that leveled a $35 million Emeryville residential complex in May 2017— the second time that site was targeted in 10 months.