The case of Satish Shrivastava

Satish Shrivastava was an old , invalid and immobile man. He was once a successful chemist and had a lot of patents and discoveries to his credit. But a paralytic attack ruined him and made him immobile. However his hands were reasonably healthy. He had been provided with a manservant Ramu by his past research organization to take care of him. Ramu had Down's syndrome and had little intelligence. The residence of Satish Shrivastava had webcam monitored by Able Security Agency. Gaurav Patekar was the president of Able Security Agency.
Satish Shrivastava was found poisoned on a Monday and he had died. He was found dead at 8:30 pm. Poison had entered into his body at 7:30 pm.
Inspector Daniel Paul started his investigations. He started by looking at webcam footage of the fateful evening. Unfortunately for some seemingly suspicious reason webcam footage from 7 pm to 8:30 pm was not available. The footage was just dark. The 6:30 pm footage showed Ramu asking the permission of Satish Shrivastava to go to market and buy rations.  Satish Shrivastava asks Ramu to keep the bottle of water nearer and gives permission to leave. The 8:30 pm footage showed Ramu crying near his master's dead body. Inspector Daniel Paul discovers the poison was in the bottle of water near Satish Shrivastava.
Who killed Satish Shrivastava ?

The case of Samir Sirohi

Samir Sirohi was lying dead in his bed. He was one of the richest man in his country and he had no sons or daughters. In fact he did not marry in his life. He had just one younger brother Saurab Sirohi.
Samir was poisoned, the doctors said. He had one known sworn enemy Lakhanpal Singh who was his rival businessman.
The police headed by Paul Rogers began to investigate and search the residence of Samir Sirohi. They found nothing incriminating. No documents, no poison bottle or anything. They enquired all the staff and Saurab too. But they did not get anything substantial. The police went back contemplating.
Two days later, Saurab rings Paul Rogers and enquires about the case. On knowing that the police were unable to go further, Saurab gets angry and asks Paul Rogers to make a more thorough search of the residence of Samir.
The police were in low morale and do as asked by Saurab. They search the residence of Samir again. This time they discover a letter written by Lakhanpal Singh to Samir Sirohi requesting Samir to allow Lakhanpal Singh to meet him.
Who killed Samir ?