Two former soldiers who killed themselves within weeks of each other should have received more support, the brother of one of the men has said.

John Paul Finnigan and Kevin Williams served together on the frontline in Iraq and formed an “undeniable bond”. When they left the army they both struggled with post-traumatic stress disorder.

In March, Williams took his own life at the age of 29. Twelve weeks later Finnigan, 34, from Merseyside, also killed himself.

Finnigan’s brother Steven, 28, said: “He joined the army because he wanted structure in his life. But when he was discharged he used to taxi injured soldiers to hospitals all around the country. It just tormented him. Not enough was done to help him.”

Williams and Finnigan served together with 2nd Battalion The Rifles in Operation Telic 9, which involved 46,000 personnel at its peak and some of the toughest fighting in Iraq. On one occasion the pair witnessed three friends being shot by a sniper as they fought insurgents in Basra.

Finnigan was medically discharged in 2010 after suffering hearing loss when a mortar bomb exploded near him.

In a text message at one point he wrote: “I suffer from feelings of rage, anger and at my worst was close to ending my life on several occasions. I’m starting to accept that PTSD will be a part of me for the rest of my life.”

More than 300 veterans and serving personnel attended Finnigan’s funeral this week after the Royal British Legion issued a plea to show solidarity with those who had PTSD. A lone piper led the hearse carrying a white floral wreath spelling out “Daddy” as a plane flew overhead carrying a banner that read “Swift and Bold”, the Rifles’ regimental motto.

His sister Nicola, 38, said: “He thought he would die as a soldier and when he came into civilian life he couldn’t cope. He suffered night terrors. Kevin’s death really affected him. They were friends and they were happy together. It was the tipping point.”

Williams became the youngest soldier to be sent to Iraq when he was deployed on his 18th birthday. An inquest heard he killed himself near his home in Basildon, Essex, after the horrors of war left him feeling “pretty much useless”.

Speaking in a documentary before his death, he said: “Returning to civilian life was a big shock. The skills I learned were all combat-based. I was pretty much useless and felt sad all the time.”

His sister, Jennifer, a software manager from Yorkshire, said he was wracked with feelings of horror and guilt. She said: “He told me one story where he saved the lives of five of his comrades but to do that he had to kill someone with a bomb strapped to them. He couldn’t understand how he could have done that and he broke down.”

Steve Nicholls, who served with Finnigan and attended his funeral, said: “He was a strong lad and a really nice guy. But I couldn’t save him and I couldn’t save Kev Williams either.”

• In the UK, Samaritans can be contacted on 116 123 or email jo@samaritans.org. In the US, the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia, the crisis support service Lifeline is 13 11 14. Other international suicide helplines can be found at www.befrienders.org.