The boys died following a disturbance in Regency Court at around 1.30am on Sunday morning (24 October).
So far officers have seized more than 200 hours of CCTV footage and spoken to more than 20 people as part of the investigation.
Three men – two from Grays aged 19 and 20 and a 21 year-old from South Ockendon – arrested on suspicion of murder remain in custody and we are continuing to question them.
Four people – a 20 year-old man from South Ockendon, and three men from Grays aged 19, 20, and 49 – have been told they face no further action.
A 40 year-old man from Brentwood has been released under investigation.
Detective Chief Inspector Stuart Truss, who’s leading the investigation, said: “Our investigation is progressing well and we are building a picture of the circumstances which led up to the boys’ death.
“We have seized more than 200 hours of CCTV footage and are now methodically going through it.
“We believe this was an isolated incident and there isn’t a risk to the wider public. Brentwood is a safe place and incidents like this are rare.
“My team is working round the clock and you’re likely to see a police presence in the area while we continue our enquiries.
“Officers from the Brentwood Community Policing Team will continue to carry out additional patrols in the area.
“They’re there to provide reassurance so please do speak to them if you have any concerns.
“We still need anyone who has any information about what happened, who we’ve not spoken to, to come forward.”
Anyone with information on the incident should call us on 101 and cite incident 125 of October 24.
Information can also be given to Crimestoppers anonymously by calling 0800 555 111 or going to https://crimestoppers-uk.org.