(CNN) Motorcycles will roar into Memorial Day weekend rallies amid heightened tension after a biker gang shootout left nine people dead in Texas.

The massacre in Waco, which was triggered by feuding biker gangs, left the community stunned.

Law enforcement authorities are on alert following what they say are threats against officers from biker groups in the wake of the attacks.

And as motorcycle enthusiasts plan to gather around the nation to mark Memorial Day, authorities are keeping a wary eye on the events.

Biking events nationwide

In New Mexico, tens of thousands of bikers will gather in Red River. Previous rallies have included the Bandidos, whose members were involved in the Waco shootout.

"Live music can be found all over town, along with vendors carrying everything from leather and lace, to food and fun," organizers said on the event website.

Activities will run through Monday, and will include a blood drive and a military history display.

In the nation's capital, bikers will attend the Rolling Thunder run, which will feature activities such as a First Amendment demonstration, a concert and laying of flowers at the Arlington National Cemetery.

Other states such as Mississippi, Missouri and South Carolina will have similar bike shows.

In the South Carolina seaside town of Atlantic Beach, authorities told CNN affiliate WBTW that security is heightened . They declined to provide an overall number of officers expected to be there.

A tense week

The events come amid a tense week following Sunday's shootout that left nine bikers dead at a Twin Peaks restaurant in Waco.

Three or four Waco officers may have opened fire, but it's unclear whether the bikers were killed by police bullets, Police Sgt. W. Patrick Swanton said.

Members of the Bandidos who are in the military "are supplying the gang with grenades and C4 explosives" to target law enforcement, according to a bulletin issued by the Texas Department of Public Safety.

It warns of plots targeting high-ranking law enforcement officials and their families with car bombs. Bandidos want to retaliate against police for shooting "their brothers" as they came out of the Twin Peaks restaurant, the bulletin says.

The bulletin is based on unsubstantiated information from an informant who said he got it from Bandidos and Black Widows motorcycle gang members.

The Texas Department of Public Safety didn't immediately respond to CNN's request for comment.

How it all started

Some motorcycle groups had reserved the outdoor bar area at Twin Peaks when another biker gang showed up uninvited, Swanton said.

A quarrel in the parking lot followed, and may have involved a tiff over a parking spot or someone having his foot run over.

At first, it appeared to be a "simple fist fight," a waitress who was at the scene said.

The woman, who didn't want her name disclosed for safety reasons, said almost all the bikers were on the patio outside.

"There was a little bit of yelling," she said. "Next thing you know, you hear the first gunshot go off. ... There was a lot of screaming (inside the restaurant)."

About 170 people were arrested. Their arrest warrants state members of the Cossacks were in the Twin Peaks parking lot when the rival Bandidos arrived.

Photos: Multiple deaths in shootout between biker gangs Photos: Multiple deaths in shootout between biker gangs People in Waco, Texas, stand near the parking lot of a Twin Peaks restaurant where nine people were killed in a shooting on Sunday, May 17. At least 18 people were hospitalized as well after a melee between rival biker gangs. Hide Caption 1 of 10 Photos: Multiple deaths in shootout between biker gangs Emergency responders tend to a wounded person near the restaurant. Hide Caption 2 of 10 Photos: Multiple deaths in shootout between biker gangs People stand near the restaurant as police investigate the scene. Hide Caption 3 of 10 Photos: Multiple deaths in shootout between biker gangs Emergency vehicles and members of law enforcement gather near the scene of the shooting. Hide Caption 4 of 10 Photos: Multiple deaths in shootout between biker gangs Authorities block an access road as the investigation takes place. Hide Caption 5 of 10 Photos: Multiple deaths in shootout between biker gangs Waco police posted this photo to its Facebook page, warning the public to stay clear of the area. Hide Caption 6 of 10 Photos: Multiple deaths in shootout between biker gangs Bikers wait in a line as law enforcement officers investigate the shooting. Hide Caption 7 of 10 Photos: Multiple deaths in shootout between biker gangs Bikers wait on a hill near the parking lot of the restaurant. Hide Caption 8 of 10 Photos: Multiple deaths in shootout between biker gangs A McLennan County deputy stands guard near a group of bikers. Hide Caption 9 of 10 Photos: Multiple deaths in shootout between biker gangs A law enforcement officer walks past debris near the parking lot of the restaurant. Hide Caption 10 of 10

There were "crime scenes inside and outside" the restaurant, including in the bathroom, dining area and around the bar.

The assailants used all sorts of weapons -- brass knuckles, guns, knives and chains. When police responded, some bikers turned their weapons on them, Swanton said.

The nine men killed in the shootout ranged in age from 27 to 65.

All of them died of gunshot wounds.