In an interview at the Upfront Summit in Los Angeles Wednesday via Axios, former Twitter CEO Dick Costolo said the company has failed to prevent user abuse on the social networking platform.

“I wish I could turn back the clock and go back to 2010 and stop abuse on the platform by creating a very specific bar for how to behave on the platform,” Costolo said. “I take responsibility for not taking the bull by the horns.”

As CEO, Costolo said his focus was often split towards other responsibilities and the scale of the issue was difficult to grapple with. Costolo also proposed tackling online abuse similarly to spam, where technology companies add roadblocks to make it harder for targeted abuse to occur. Costolo was Twitter at CEO from 2010 to 2015.

In the past few years, the social networking platform has struggled to handle trolling and abusive behavior. Public figures such as Saturday Night Live cast member Leslie Jones and actress Zelda Williams have been targeted by online trolls. In both incidents, Jones and Williams were persistently harassed with tactics including racist comments, death threats and messages targeting Williams’ father Robin Williams. For other high-profile and general users, problems like stalking threats, harassment and doxxing — referring to the revealing of personal information like a home address — are similarly commonplace.

Twitter has attempted to make up ground by establishing a Trust & Safety Council last year, but its poor responsiveness to harassment and threats contributed to Disney’s decision to avoid potentially acquiring the company. While Twitter has placed a strong emphasis on its belief in free speech, the networking platform has often been paralyzed by balancing these founding values with its popularity among trolls and online abusers.

In the past, Costolo has privately agreed with Twitter’s poor reputation. Via a leaked internal post from February 2015, Costolo acknowledged the company had been slow to respond to abusive users.

“We suck at dealing with abuse and trolls on the platform and we've sucked at it for years, Costolo wrote. “It's no secret and the rest of the world talks about it every day. We lose core user after core user by not addressing simple trolling issues that they face every day.”