Under Virginia law, this is not rape, which is defined in terms of sexual intercourse.
What is sexual battery?
Sexual battery is when one
sexually abuses, as defined in § 18.2-67.10, (i) the complaining
witness against the will of the complaining witness, by force, threat,
intimidation, or ruse
and sexual abuse is
an act committed with the intent to sexually molest, arouse, or
gratify any person
("complaining witness" in fact is the person alleged to be the victim, and this does not mean that the victim must "press charges").
Is the act 'sexual abuse'?
So the first question is whether the act constitutes "sexual abuse": did someone (the violator) have the intent to sexually molest, arouse, or gratify a person. One might suppose that the assailant intended to arouse himself, or to gratify himself. And then the question is whether this means specifically "sexually arouse" and "sexually gratify", or would it be contrued as general arousal, or gratification. If the latter, it would be very hard to argue that the act was not "sexual abuse", but if the former, the counter-argument could be "I thought it was funny" or "I hated her and did it out of spite".
The scope of "sexual" is ambiguous: it could refer to just the first of the terms, or all three terms. There is no official name for the jurisprudential principle identified by Solan (The language of judges) as the "across the board rule", which is sort of the opposite of the last antecedent rule (which has you interpret ambiguous modifier-modified relations narrowly). A narrow interpretation of "sexual" (i.e. "sexual molestation, or any kind of gratification") would have the bizarre consequence that anything that makes someone happy, like telling a person a joke, constitutes sexual abuse. This is absurd, therefore the intended meaning must be "sexually molest, sexually arouse, or sexually gratify". Prosecution might procede on the premise that the assailant intended to receive sexual gratification from the act. Or, it could proceed on the premise that the act is sexual molestation.
What does it mean to 'sexually molest'?
The statutes do not define "sexual molest". Jurors could make the determination in one of two ways. First, there could be a jury instruction that defines the phrase (presumably derived from distillation of prior case law); second, they could assign the expression the "plain meaning". The danger here is that the plain meaning of "molest" is based on an effect on the victim, which arguably does not exist in this case. However, most people would feel that this is sexual molestation.
The instruction spelling out the definition of sexual abuse is slightly different from what the statute says, but does not flesh out "sexually molest":
Sexual abuse means an act committed with the intent to sexually
molest, arouse or gratify any person, where the defendant
intentionally touches the complaining witness’ intimate parts or
material directly covering such intimate parts.
It should be noted that ordinary consensual sex is, under Virginia law, "sexual abuse" (which itself is somewhat absurd, and will certainly be confusing). Sexual molestation is not a crime: sexual abuse is.
What is the difference between sexual abuse and sexual molestation?
"Abuse" adds to the meaning of "molestation" that "abuse" is "against the will of the complaining witness, by force, threat, intimidation, or ruse". The later 3 conditions are clearly not applicable. Is the act "against the will", or "by force"? Neither expression is given a legal definition. Again, these terms are taken to be self-evident, and the jury instructions do not clarify what constitutes "against the will" or "by force". The question would arise whether "against the will" specifically mean "contrary to the will", or "lacking a will to do so". Force in the broadest sense includes any kind of touching.
A minor complication is that the law defines the crime in terms of "against the will of the complaining witness, by force, threat, intimidation, or ruse", which could be interpreted as "any of these five reasons". Or it could be interpreted as "against the will when it is by one of these four means". The second interpretation would mean that the state has more things to prove and fewer choices, the former means it has only one thing to prove and more choices. Because of the comma, this is to be interpreted as a list of 5 things. If the statute said "against the will of the complaining witness by force, threat, intimidation, or ruse", it would mean "by force which is one of the following four".
What about assault (or battery)?
There is no crime of "sexual assault" in Virginia (it is subsumed under various other names). There is a crime labeled assault and battery, which says
Any person who commits a simple assault or assault and battery is
guilty of a Class 1 misdemeanor
That means that battery alone is not a crime: the crime involves at least assault. There is no further statutory explication of "assault" and "battery", but the instructions say that
An assault is [an overt act intended to do bodily harm to another
together with the present ability to cause such harm; an overt act
intended to place a person in fear or apprehension of bodily harm that
creates in (him;her) a reasonable fear or apprehension]
Arguably, groping is not intended to bodily harm another. The tort law understanding of battery is a bit different (often conflated with assault), since it also includes "offensive contact", and "harmful" and "offensive" contact are not the same. Virginia omitted the "offensive contact" part of the concept. In tort law, "harm" includes "physical harm" (which is "physical impairment") and other kinds of harm (i.e. detriment). Lewd touching is not physical harm, but it is harm. It's uncertain whether it is bodily harm (harm with respect to the body? or a rewording of "physical impairment"?), using Virginia's terms.
Since the require requires at the minimum that there be an assault, and since offensive contact is not assault, we should stop here.
Still, you might want to know about "battery" since that might cover the situation you're interested in. The term "battery" is explained in the jury instructions
A battery is the willful touching of another, without legal excuse or
justification, done in an angry, rude, insulting or vengeful manner
This does not require that the victim have any knowledge of the act. Unsurprisingly, the law does not define what constitutes "in an rude manner", however people would ordinarily interpret groping without consent as "rude". However: simple battery is not a crime. There are certain special conditions where battery (without assault) is a crime: battery against a health care provider, battery against a full-time or part-time employee of any public or private elementary or secondary school on the job.
Under the tort version of the "interest in freedom from offensive contact contacts" (see §18, Restatement, 2nd) illustration 2 says
A kisses B while asleep but does not harm her. A is subject to
liability to B.
In the present case, the accused is liable for the tort of battery. But criminal law is different.
So, on a narrow reading of Virginia (criminal) law, the investigator is apparently correct.