Student Ricky Reel was 20 when he vanished during a night out in London, shortly after he and his friends had been racially abused by two white men.
His body was found in the River Thames.
The man tasked with investigating undercover policing did not name specific families but criticised the “routine gathering” of information.
Mr Reel’s mother, Sukhdev, said she had been informed that officers gathered intelligence on her in 1998 and 1999.
‘Time to grieve’
She wants a public inquiry into the spying claims and told the BBC: “This was happening at a time we were feeling very low, we should have been left alone to grieve for our son, instead of being spied upon.”
Mr Reel, a student at Brunel University, was last seen alive on 15 October, 1997, after a night out with three friends in Kingston upon Thames, south-west London.
Two white youths had attacked the group of young Asian men. As his friends fought them off, Mr Reel disappeared.
His body was found a week later.
Police said his death had probably been an accident, and an open verdict was recorded at an inquest in 1999.