Math 175: The Hidden Message

Below is a bit stream of length 1000. The bits in this stream were generated randomly up until some point when a 'non-random event' took place over part of the bit stream. After this event concluded, the bits were once again generated randomly.

Determine, to within 20 bits, the starting point and finishing point for the event. Explain how you came up with your answer.

11100001110000011010100010100000111001101100001010
11011101110100011001101011011110111101101001101111
11011001011111101100001110110110110011011010011001
10111001111011101001010111010100111111101000101000
11111010110010000001101100010100100111001010000001
00000001100011100001000011010000001100100001011001
10010010100010111101110000111111000110101011111010
10110110000001011100110111111111110111100101100011
11011111010001011000110010001011001101101110001011
11000100011110001000100010010111111101100001101110
11110010100000101001011010000010010011110000110110
00010101100011001001001010000001101001010001101011
01001101010000001100101100010100010000100000000100
01000100000000001000001100110010011001100111011101
11010001000011111011111011000000000101101110111001
11011110111010110111110011101000110011001000111100
11011000101000101111101111110010010011101111101110
11010001011011000101010110010111000000001010101110
11001010000001000001101111000100100110110111010000
10000111010101001111110010110000011111101110101111

Another bit stream of the same length is to be studied for a lab report (the assignment is the same as above):

11110101010000101011000111101111001011011110100100
00100000011010101010001100111000111101100110111000
01011100000000101010001011110111101010111011010011
10000010111110111001101011100110011000011100100100
01100010111010011111011010000011110111111010000100
00011100100111100010101000011010010000001001100011
11001001110001110101100011101000001001101100110110
00011001010111011011100011000101010000100110011110
10100110011101010011101111000001110100111011011010
01111000111111010101011000111110001110010001101000
10011101000001110001010110010110110110110110101000
11110010011000110110111000011110011110000001111000
00011111010010100101101111100000001111011000011111
10000100101100111000100110011000111111001000010111
01010110001110000010001010110110001110101111111011
01011100100010100100011100100110110100111001111101
11101111111000011010000011011110001111111001001111
10001110001011101011000001010000001111111011010000
10101000100101011110001010000101010100010111110010
00100000111101011010110111001101110011100011010111