The mother of Aaron Alexis, the former navy reservist who killed 12 people in a shooting spree at the Washington navy yard, made an emotional statement on Wednesday, saying she was heartbroken for the families of the victims.

Speaking from her home in New York, two days after the incident, Cathleen Alexis said she was glad her son was "now in a place where he can no longer do harm to anyone".

More details emerged of Alexis's recent mental health problems. In early August, Alexis, 34, called police in Rhode Island, saying he thought people were following him and talking to him through the walls of his hotel room. Police informed the local navy authorities, but it appears no attempt was made to revoke his access to military installations.

Cathleen Alexis did not address her son's state of mind in her short statement on Wednesday. She told a CNN reporter: "Our son, Aaron Alexis, has murdered twelve people and wounded several others. His actions have had a profound and everlasting effect on the families of the victims.

"I don't know why he did what he did, and I'll never be able to ask him why. Aaron is now in a place where he can no longer do harm to anyone, and for that I am glad. To the families of the victims, I am so, so very sorry that this has happened. My heart is broken."

CNN did not broadcast video of the statement. On Tuesday, Cathleen, who lives in the Bedford-Stuyvesant neighbourhood of Brooklyn, New York, told police she was willing to talk to one reporter but did not want to be filmed or photographed.

In Washington, Barack Obama, voiced exasperation over the "ritual" of mass shootings in the US, which he said routinely shock the nation but then fail to convince Congress to initiate even rudimentary gun control laws.

Speaking to the Spanish-language network Telemundo, he said that it was up to legislators to take action to prevent a repeat of the navy yard shooting.

Obama said: "The fact that we do not have a firm enough background check system is something that makes us more vulnerable to these kinds of mass shooting.

"I do get concerned that this becomes a ritual that we go through every three, four months, where we have these horrific mass shootings. Everybody expresses understandable horror. We all embrace the families and obviously our thoughts and prayers are with those families right now – as they're absorbing this incredible loss.

"And yet we're not willing to take some basic actions that we know would make a difference."

Obama said he had "put in place every executive action" that he proposed after the mass shooting at Sandy Hook elementary school in Newtown last December, but restrictions on access to firearms needed to be implemented by Congress.

The Obama administration has announced three separate reviews prompted by the navy yard shootings, including one into the procedures governing security clearances for contractors such as Alexis.

A handful of lawmakers seized on the shootings at the navy yard, which is less than two miles away from Capitol Hill, to renew calls for gun reform. But the consensus in Washington is that any legislation is highly unlikely to pass the Senate or House of Representatives.

It has emerged that Alexis, 34, bought a shotgun and ammunition from a store in Virginia 48 hours before the killing spree, despite a history of violent outbursts and two firearm-related arrests.

In the latest incident, Alexis told police he got into an argument with someone as he was getting on a flight from Virginia to Rhode Island, where he was working as a naval contractor, and he said the person sent three people to follow and harass him.

On 7 August, about a month into a new job doing navy contract work, Alexis called police while he was staying at a hotel in Newport, Rhode Island. Two officers were dispatched, who reported that Alexis told them heard voices talking to him through a wall while at one hotel, so he changed hotels twice.

However, he said the voices followed him and he feared they might harm him. He also though that vibrations were being sent into his body by a microwave machine to deprive him of sleep.

Newport police lieutenant William Fitzgerald told the Associated Press that the Rhode Island naval station was alerted and sent a copy of the police report later the same day. A spokeswoman for the naval station did not comment.

Concern over the ease with which Alexis acquired a firearm and was able to retain his security clearance to work on navy installations has intensified over the past 48 hours. It previously emerged he had been arrested three times, on two occasions after incidents involving guns. In one, in Seattle in 2004, he shot the tyres of a construction worker's car.

In another, in Texas in 2010, he fired through the ceiling into the apartment of his upstairs neighbour's apartment a dispute over noise. Neither resulted in a prosecution.

Alexis, who was honourably discharged from the navy in 2011 following a pattern of concerning behaviour, was reportedly undergoing mental health treatment from the Department of Veterans Affairs since August. © Guardian News and Media 2013