Political extremists, criminals and gang members are advertising their wares, flaunting their exploits and recruiting new members in 140 characters or less, a report by the American USA Today cited enforcement authorities, criminologists and security experts.

Extremists spread their propaganda via video. Gangs post their colors, signs and rap songs to showcase their criminal enterprises. Prostitutes and drug dealers troll for new customers. Teens trash a former NFL player's house and brag about it with photos on Twitter.

Extremist groups, domestic and international, have been particularly savvy in their use of social media, says Evan Kohlmann, a senior partner with the security firm Flashpoint who specializes in the online communications of extremist groups. Twitter has become their site of choice because it is easy to sign up and remain anonymous among millions of users and tweets.

"These groups realize they need to reach as many people as possible," he says. "And Twitter and Facebook is where you find people."

In the beginning, extremist groups were reluctant to use social media. They relied on password-protected online forums, Kohlmann says. But as social sites became ubiquitous, the groups and their members jumped in like everyone else, he says.

One of the early and most prolific outfits to turn to Twitter was al-Shabab, the radical Somali group with links to al-Qaeda whose name means "The Youth" in Arabic.

Al-Shabab used Twitter during the hostage siege at the Westgate Shopping Mall in Nairobi to ballyhoo the mayhem blow-by-blow. Tweets defended the attack, mocked the Kenyan military and president, posted photos of members inside the mall and threatened more bloodshed.

Twitter shut down at least five different accounts used by al-Shabab. But each time the microblogging site suspended an account, the group created another with a different user name.

Twitter says it doesn't comment on individual accounts for security and privacy reasons.

Al-Shabab currently has a working feed on the site. Since Wednesday, the group posted audio statements by its leader, Ahmed Abdi Godane, also known as Mukhtar Abu Zubair, justifying the siege and threatening more attacks. It posted a tweet accusing the Kenyan government of demolishing the mall intentionally: "To cover their crime, the Kenyan government carried out a demolition to the building, burying evidence and all hostages under the rubble #Westgate."

The Associated Press reported Friday that the military caused the collapse, citing an unnamed senior Kenyan police official. The official said Kenyan troops fired rocket-propelled grenades inside the mall, but would not say what caused the collapse or whether it was intentional.

J.M. Berger, a security analyst who is editor of Intelwire, an online magazine that monitors extremist activity, says it's not the first time the group has tweeted about its activities in real time. He says Al-Shabab used Twitter to broadcast messages and threats during bombings in Mogadishu and to trumpet an attempt to assassinate the president of Somalia in early September.

"This time was more visible simply because the attack itself was more visible and unusual in its nature," Berger says.

A growing number of homegrown extremists are also turning to Twitter.

A May 2013 report on digital hate speech from the Simon Wiesenthal Center says Twitter helped spur a 30% growth in online forums for hate and terrorism over the past year. The study says more than 20,000 "hate-spewing hashtags and handles" appeared on Twitter in 2012, up 5,000 from the year before. The group identified Twitter as a "chief offender" among social media sites because of a lack of monitoring of hate and terrorist content.

Those who monitor extremist activity online say that as the site of choice for extremists, Twitter needs a clear, transparent policy as to what content is off-limits, and it has to enforce that policy vigorously.

"They respond to abuse reports, but their criteria for suspension is very limited," Berger said in an e-mail interview. He spotted al-Shabab's tweets during the mall siege and notified Twitter.

"They are broadly permissive of extremist content in a way that other services, like Facebook and YouTube, are not," he says.

Twitter, through spokesman Nu Wexler, would not make anyone from the company available for an interview. He directed a reporter to a blog post by the company's head of safety, Del Harvey, who wrote that manually reviewing every tweet is simply not possible. Users post up to 500 million tweets a day in more than 35 languages.

"We use both automated and manual systems to evaluate reports of users potentially violating our Twitter Rules," Harvey wrote. "These rules explicitly bar direct, specific threats of violence against others and use of our service for unlawful purposes, for which users may be suspended when reported.

Al-Qaeda-linked groups are also highly active on Twitter regarding the war in Syria.

They have been able to absorb hundreds of people from across Europe and North Africa to fight against the government in the Arab country.

A recent study by the British defense consultancy showed that militants fighting against Syria now number around 100,000 fighters, which are fragmented into around 1,000 bands.

SHI/SHI